Indoor position location using delayed scanned directional reflectors

ABSTRACT

A mobile device determines its location accurately by measuring the range to a position reflector as well as azimuth and elevation angles of arrival (AOA) at the reflector. The mobile can transmit a coded radar signal and process reflections to determine its location. The reflectors may include internal delays that can identify the reflector and provide transmit/receive separation for the mobile. The reflection can include a primary and further delayed secondary reflection. The mobile can determine the internal delay of the reflector based on the delay between primary and secondary reflections. The range and AOA information can be combined with information about the position, orientation, and characteristics of the reflectors to determine location. In some systems, the mobile device can determine its location in a three-dimensional space using reflections from only one reflector. The reflectors, which can be economically produced, can be unpowered and low profile for easy installation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 62/041,569, filed Aug. 25, 2014, which is hereby incorporated byreference.

BACKGROUND

The present invention relates generally to the field of positioning and,in particular, to systems and methods suitable for indoor locationpositioning.

Indoor position location is becoming increasingly important in today'smobile wireless devices environment. Whereas it took some time foroutdoor location services to become widespread after the technology wasavailable, today the applications for indoor position location is racingahead of available technical solutions. Many technologies have been usedfor outdoor position location, chief among them are the GlobalPositioning System (GPS) using orbiting satellites that transmit timingand ranging signals that enable a receiver to calculate its positionaccurately. In addition to GPS a number of other technologies have beenproposed with various degrees of adoption including cellularbase-station triangulation and trilateration.

Unfortunately, technologies employed for outdoor position locationperform poorly for indoor position location. This is because the signalof these systems is either missing or very weak indoors, as in the casewith GPS, or the signal arrives with significant multipath to render itinaccurate for indoor positioning.

Some proposed systems attempt use available indoor Wi-Fi wirelesssignals to aid in determining the indoor position of a wireless portabledevice. Unfortunately, Wi-Fi radio-frequency (RF) environments canchange unpredictably over time and even under ideal conditions thelocation may only be accurate to a resolution of a few meters. Otherproposed systems employ wireless beacons that use the Bluetooth wirelessprotocol to narrow down the location of a wireless device. Such systemsrely on detecting proximity to one of such beacons and thereforerequires deploying a different beacon near every point of interest inthe indoors environment. Only a few feet away from a beacon, theestimate of the position is no more accurate than one using Wi-Fisignals. These systems rely on detecting a certain router or a beaconwith a certain preregistered code associated with a known and publishedphysical location to establish approximate location and then use aReceived Signal Strength Indicator (RSSI) to further narrow down thelocation. Such methods are limited in the ultimate resolution they canachieve in a large indoors area. Also while the first relies on alreadyavailable low accuracy and often varying Wi-Fi signals, the beaconmethod requires blanketing an indoor area, such as a department storefor example, with a large number of active beacons that need constantpower source and physical management as the store changes promotions andrefashions internal displays. In addition, those methods are either tooinaccurate or too cumbersome to adopt in a domestic residentialenvironment. Furthermore, since such methods rely on RSSI measurements,signal penetration through walls and floors results in a major drawbackfor both commercial as well as enhanced E911 services since with walland floor penetration the location of an individual cannot be narroweddown to an exact room or an exact floor.

Other proposed systems for indoor position location are based onUltra-Wide Band (UWB) methods, for example, as described in“Ultra-Wideband Positioning Systems” by Sahinoglu, Gezici and Guvenc,2008. Unfortunately, such systems depend on a powered activeradio-frequency identification (RFID) tag for proper operation and mayinclude bulky batteries. With continuous tracking of numerous userswithin some vicinity, battery life becomes impractically short. Inaddition, the RFID tags may be costly.

Other proposed systems use coded LED lighting to determine indoorposition location. Such methods rely on the camera of a mobile devicedetecting a coded lighting flicker from an LED light fixture todetermine location. Unfortunately, there is a significant time where themobile device of an individual is placed in a carrying bag or in apocket. Also, in most cases, a costly replacement of numerous lightingfixtures is needed to accommodate the new LED bulbs. Furthermore, theaccuracy of such technologies is only able to provide approximatelocation, such as location within a room, rather than determine actuallocation with sufficient resolution for commercial and residentialapplications demand accuracy on the order of few centimeters. Similarlimitations occur when using infrared beacons.

Other proposed systems use ultrasonic sound waves. Such systems requireretrofitting indoor spaces with multiple wireless/ultrasonic beaconsand/or receivers needing their own power sources. In addition,ultra-sonic transmitting and receiving transducers may be too bulky andconsume too much power for a mobile device or a battery operated beacon.In addition, ultrasound is severely muffled in a person's carrying bagor inside a pocket.

SUMMARY

In one aspect, a positioning system is provided that includes: one ormore position reflectors, each of the position reflectors configured toreflect radar signals, each reflected radar signal including a primaryreflection delayed by an internal delay of the position reflector; aserver storing reflector data associated with the one or more positionreflectors; and a mobile device configured to receive the reflector datafrom the server and to transmit a radar signal and process reflectionsof the transmitted radar to determine a location of the mobile device.

In another aspect, a method for location positioning of an electronicdevice is provided. The method includes: transmitting a direct sequencespread spectrum coded radar signal, the radar signal having a centerfrequency swept between two frequencies; receiving radar signals thatinclude reflections from a plurality of position reflectors, each of thereflections including a primary reflection delayed by an internal delayof the corresponding position reflector and a secondary reflectionfurther delayed by the internal delay of the corresponding positionreflector; correlating information detected in the received radarsignals with corresponding information used to form the transmittedradar signal to determine maxima in the reflections; detecting frequencyinformation and delay information about the reflection using thedetermined maxima in the reflections; determining information about theinternal delays of the position reflectors from the delay information;identifying the plurality of position reflectors using the determinedinternal delay information; determining a range to at least one of theplurality of position reflectors using the detected delay informationand the determined internal delay information; determining an angle ofarrival for the reflections from at least one of the plurality ofposition reflectors using the frequency information; and determining theposition of the electronic device using the range and the angle ofarrival.

In another aspect, a device for location positioning is provided thatincludes: a radio-frequency front end coupled one or more antennas; anda processor coupled to the radio-frequency front end and configured tosupply a radar signal to the radio-frequency front end for transmission,receive, from the radio-frequency front end, signals reflected from oneor more position reflectors, and process the received signals todetermine a location of the device transmit, the processing includingdetermining a delay from a primary reflection to a secondary reflection.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of the present invention, both as to its structure andoperation, may be gleaned in part by studying the accompanying drawings,in which like reference numerals refer to like parts, and in which:

FIG. 1 is a functional block diagram of an indoor positioning systemaccording to a presently disclosed embodiment;

FIG. 2 illustrates an example location positioning according to apresently disclosed embodiment;

FIG. 3 illustrates another example location positioning according to apresently disclosed embodiment;

FIG. 4 is a functional block diagram of a position reflector accordingto a presently disclosed embodiment;

FIG. 5 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment;

FIG. 6 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment;

FIG. 7 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment;

FIG. 8 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment;

FIG. 9 is an isometric diagram of a position reflector according to apresently disclosed embodiment;

FIG. 10 is a plan view of a position reflector according to a presentlydisclosed embodiment;

FIG. 11 is a plan view of another position reflector according to apresently disclosed embodiment;

FIG. 12 is a functional block diagram of transmit and receive radarchains according to a presently disclosed embodiment;

FIG. 13 is a functional block diagram of further transmit and receiveradar chains with an alternative antenna arrangement according to apresently disclosed embodiment;

FIG. 14 is a functional block diagram of further transmit and receiveradar chains with another alternative antenna arrangement according to apresently disclosed embodiment;

FIG. 15 is a functional block diagram of location signal processingaccording to a presently disclosed embodiment;

FIG. 16 is a functional block diagram of transmit location signalprocessing according to a presently disclosed embodiment;

FIG. 17 is a functional block diagram of receive location signalprocessing according to a presently disclosed embodiment; and

FIG. 18 is a flowchart a process for determining position locationaccording to a presently disclosed embodiment.

DETAILED DESCRIPTION

Systems and methods through which portable wireless mobile devices candetermine their indoor position accurately, e.g., to centimeter levels,using radar reflectors are disclosed. Frequency, and/or coded signal,scanned retro-directive or retro-reflective passive radar reflectorswith built in fixed RF delays can be used. Such reflectors can be placedsomewhere on the ceilings, walls, or floors of a given indoor space andtheir locations and orientations made available to the mobile device.

A mobile device can determine its location accurately by measuring thedistance between it and a given nearby reflector as well as the azimuthand elevation angles of arrival (AOA) at the reflector using a speciallycoded radar waveform. By being able to determine its distance and anglesrelative to a given reflector of known location and orientation, themobile device is able to determine its location in a three-dimensionalspace using only one passive reflector. Detecting reflections frommultiple reflectors can increase the computed accuracy. Furthermore,carrier phase range determination techniques can be employed to yieldimproved accuracy under various indoor environments. Finally, given thereflectors' passive nature and low profile design, their installation issimple in both commercial and residential settings, for example, byaffixing them to any flat and unobstructed surface within the indoorspace or placing them on top of a ceiling tile with no wiring orsurveying. Once installed, no maintenance is needed, such as batteryreplacement or recalibration. Furthermore, such reflectors contain veryfew passive components with lax tolerances, and therefore are veryinexpensive to produce. These reflectors are also typically flat andunobtrusive and can be permanently installed on, behind or built intothe drywall of a building for example.

Reflectors used with the presently disclosed methods and systems havevarious forms and functions. Some reflectors provide ranginginformation. Other reflectors provide angle information such as azimuth,elevation, or both. Reflectors may also provide both ranging and angleinformation. The reflectors may be referred to as radar reflectors,position reflectors, location reflectors, or similar terms.Additionally, reflectors may be referred to using adjectives thatdescribe characteristics of particular reflectors, for example,composite reflector, delayed reflector, frequency scanned reflector,frequency scanned retro-directive reflector, unscanned retro-reflectiveradar reflectors, horizontal sweeping reflector, vertical sweepingreflector, linear reflector, passive reflector, or sectored reflector.

In an embodiment, a mobile device includes a circuit that transmits aradar signal to be reflected by a wall or ceiling mounted RF reflector.The reflector may include an internal delay. The reflector may be afrequency-scanned phased array type antenna structure. A frequencyscanned phased array antenna structure is an antenna structure that hasa high gain lobe in a given direction based on the frequency ofexcitation (see, e.g., “Antennas”, second ed., John Kraus, 1988, section11-11). As the frequency of excitation is changed, the directionalangles of this main high gain lobe of the antenna also changes. Similarantenna structures have been used in radar installations where bysweeping the excitation frequency, the structure is made to sweep thedirection of concentrated radiation angle out of the structure. Giventhe reciprocal nature of antenna structures, the direction along whichthe radiation is maximized for a given frequency is also the directionof maximum reception sensitivity at the same frequency. These types ofantenna structures have only one connection port even though they aretypically made out of a phased array antenna matrix. The phasing of theelements is achieved by inserting phasing delays in the RF powersplitting/combining branching traces from the single port of the antennastructure to the radiating elements within the structure. Positionreflectors can be installed in the ceiling corners of a room. In suchcases, the required sweep angle can be less than 90 degrees. In oneembodiment, these reflectors are made in such a way so as to havenon-zero gain only in the direction of the room, without significantbackwards sensitivity. This can reduce the possibility of reflecting orinteracting with signals from adjacent rooms or floors.

In another embodiment, the reflector is designed to yield both azimuthand elevation AOAs. A single linear phased array can sweep an arc alongone angle. To enable a reflector to yield two angles, a compositereflector may include two collocated reflectors, one sweeping verticallyand the other horizontally. Each of the collocated reflectors respondsto a different sub-band of excitation. The mobile transmits a frequencysweep for each sub-band and separately detects the azimuth and elevationAOAs. Some antennas can produce a raster scan as a function ofexcitation frequency and hence are able to yield azimuth and elevationangles with one frequency sweep.

In an embodiment, a mobile device transmits a radar signal with acontinually sweeping center frequency. At some instant, the frequency ofthis radar signal equals the frequency that results in maximum antennagain in the direction of the mobile device. At this point, the mobiledetects maximum reflected signal power from the reflector back to themobile. By noting this frequency and using the information that themobile already has from the network about the position, orientation andangle of radiation as a function of frequency for this antennareflector, the mobile is able to compute the angle relative to theantenna structure along which the mobile resides. In addition, bymeasuring the round trip delay between the transmission of the signaland subsequent detection of the reflection, the mobile can also computethe distance between the mobile location and the reflector. Havingknowledge about the location and orientation of the reflector and havingdetected the distance and AOA to this reflector, the mobile is able tocalculate its position in 3D space. In order to achieve very highaccuracy, carrier phase ranging techniques can be used.

In one embodiment, the mobile transmits a radar signal that sweeps acertain width of frequency looking for the highest reflected power. Thefrequency with the highest reflected power is the frequency resulting inmaximum reflection in the direction of the mobile. At this point, themobile may limit the frequency sweep to frequencies close to this centerfrequency for subsequent position fixes since the mobile may not havemoved much. This reduces the search time and reduces transmitted power.Using power control in the mobile, that is by reducing or increasing theradiated power for a given radar signal to transmit a signal thatresults in reflections with minimum detectable reflected power, themobile can increase battery life and reduce interference to other closeby devices. In addition, two mobiles that are separated by a given anglerelated to the reflector would be using different frequencies for theirradar signals since their AOA to the antenna is different. Thisincreases the capacity of the system since one reflector can handlemultiple mobile devices within a given area simultaneously and withminimum cross interference because of differing frequencies and antennagain sectorization based on the reflector antenna retro-directiveradiation concentration towards the direction of the exciter. In thecase of mobiles that are very closely collocated, time division,frequency division or code division multiple access techniques could beused so that each device could distinguish its own signals from that ofothers.

In one embodiment, the signal received at the reflector antenna isdelayed before being reradiated back to the mobile. The delay istypically on the order of a few fractions of a microsecond to severalmicroseconds but could be larger or smaller. This could be achieved by anumber of passive RF components such as, bulk acoustic wave (BAW)devices, loaded delay lines, LC delay lines, surface acoustic wave (SAW)devices or other RF-Acoustic methods (e.g., similar to those previouslyused by portable video cameras to support the PAL TV standard). Inaddition, a signal could be delayed by using newly developedmeta-material structures with negative permittivity and/or permeabilitywhere group delays could be made arbitrarily long.

In the case of an embodiment using a BAW filter, the input of theantenna feed line is matched and connected to one side of the BAW filterwhile the other side of the filter is shorted to ground. The BAW filtermay be selected to pass all of the energy in the radar transmittedsignal. A signal received at the antenna has an unavoidable immediatereflection due to unavoidable mismatches. Then the bulk of the signal isfed into the BAW filter. After one propagation delay time through theBAW filter the signal is fully reflected by the short to ground. Thesignal experiences another propagation delay time before driving theantenna structure and reradiating back in the direction of the mobiledevice. In this embodiment, the mobile knows a priori this delay of agiven reflector and subtracts it from the round trip delay calculationin order to calculate the true physical distance between a mobile andthis reflector.

By delaying the radar pulse before reflecting it back to thetransmitting mobile a number of advantages are achieved. Firstly, atransmitted radar signal from a mobile may have about a 90 ns delayspread for a typical room to 200 ns delay spread for a shopping center.This is the time during which the signal continues to persist withdetectable power due to parasitic reflections from environmentalobstacles (see, e.g., “OFDM For Wireless Multimedia Communications”, R.Van Nee and R. Prasad, 2000, sec. 1.3). By delaying the received signalat each reflector by a time longer than this time, false locationdeterminations due to parasitic environmental reflections, such as frommirrors or electrically large metal objects, are eliminated since asignal received after a delay longer than the minimum reflector delaycan only come from a reflector. It is possible though, to still get areflected signal from a reflector that also reflects off of a conductingobject along its way to a mobile. This multipath parasitic signal has amuch lower probability of occurrence than direct parasitic reflectionsdue to the delay in the reflectors and its effects can be furtherreduced by making sure that the reflector is placed in a location thatmaximizes the probability of line of sight reflections. Also, in mostcases, large parasitic reflectors are located on or near walls. Becausethe mobile also knows the angle of incidence at the reflector, andbecause such reflectors are close to room surfaces, an erroneouscomputed position typically leads to illogical physical locations due toroom extents and can be dealt with accordingly.

In addition, an internal delay inserted in the reflector allows themobile to transmit a radar wave that is much longer, when mapped inphysical space using the speed of light, than the typical maximum indoordistance between a mobile device and a reflector while at the same timestill allowing for the transmitted and reflected signals not to overlapat the mobile. A longer radar signal results in higher distanceresolution and better lower peak to average ratio for the transmitter.The non-overlap also means that the mobile can allocate alternatenon-overlapping times for either signal transmission or reflectionreception modes. By scheduling the transmit and receive modes ofoperation at alternate non overlapping times, the receiver front-endcircuit is greatly simplified since TX signal leakage back to the RXside are avoided due to TX and RX not occurring simultaneously.

In addition, the radar signal may be encoded using direct sequencespread spectrum (DSSS) phase modulation. In these systems, the internalreflector delay facilitates distinguishing between a transmitted signaland its reflection if the built in reflector delay ensures a more thanone chip delay separation.

The delay of the signal within a given reflector can be known andtightly controlled on a continuous basis. This is because this delaydirectly adds to the round trip delay and any discrepancies between itspreviously published delay and the actual delay, due to temperaturedrift or component aging, results in direct error in computing thedistance between a mobile device and a reflector. While it is possibleto continuously measure and republish the actual delays in a timelyfashion by a periodic calibration process, an alternate embodimentdisclosed herein addresses this in an alternate fashion.

In one embodiment, the reflector uses a BAW filter to delay the receivedradar signal. Instead of reflecting all the power, the reflectorinternally uses a directional coupler that matches the impedance of theantenna structure to the BAW device for a received signal. The signalwithin the BAW having been reflected by the short to ground on the otherside of the BAW is reflected towards the input port of the BAW. In thisdirection, an intentional mismatch exists that allows a portion of theoriginal signal to couple back to the antenna structure and get radiatedback to the mobile while the remaining portion is reflected back intothe BAW. A portion of this redirected remaining portion is allowed tocouple to the antenna after another round trip delay within the BAW andthe cycle repeats. For example in one embodiment, the mismatch wouldallow ⅘, or −1 dB, of the original signal to couple to the antenna andcycle back one-fifth. On the second reflection, the antenna wouldreradiate ⅘×⅕= 4/25 or about −8 dB of the original signal. Nextreflection would be ⅘*⅕*⅕= 4/125 and so on. The important fact here isthat the mobile shall receive a portion of the signal after 2 times theBAW delays plus free air round trip delay. It shall receive anothersignal later on at precisely 4 times the BAW delay plus the same freeair round trip delay. Given these two readings, the mobile canaccurately measure the BAW round trip delay, subtract it from themeasurement of the first reflection round trip delay and hence canaccurately determine the distance from the mobile to the reflector.Since this is done on the fly, no calibration, matching or measurementsneeds to be repeated after the installation of the reflectors since themobile is able to adjust for the possible small delay drifts due totemperature and component aging. Manufacturing tolerances imposed on BAWgroup delays could be relaxed significantly and the published reflectordelay need only be approximate. A design tradeoff involves choosing whatfraction to couple back to the antenna and what portion to reflect backto the BAW. Having a large portion couple to the antenna increases theSNR of the first reflection but subsequent reflections could be too weakto detect. In contrast coupling large portion back to the BAW weakensthe first reflection and the structure continues to re-radiate residualenergy for a long time. For example if ¼ is chosen, the first reflectionfrom the antenna would be −1.25 dB down from the original receivedpower. The second reflection would be about −7 dB down from originalreceived power and the third is −13.25 dB and so on. Alternatively if1/10 is used, the reflected signal powers would be −1 dB, −10.45 dB and−19.9 dB respectively. Please note that the mobile needs to process thesecond reflection only once, after that the mobile need only process thefirst reflection since the delay in the BAW is not expected to changeappreciably in a short period of time. Furthermore a mobile devicehaving performed this reflector delay determination with a strong degreeof confidence uploads this measurement value along with a degree ofconfidence indicator to the public server to be used by other mobiles.Depending on the degree of calibration confidence, other mobiles mayelect to use this recently published value rather than repeat themeasurement themselves. Please note that we have ignored delay elementinsertion loss for clarity. In practice, the reflected signals wouldincur two insertion losses of the delay element for each round trippropagation within the delay element. We also have assumed that a BAWfilter is used for the delay as an example. Other delay elements mayalso be used.

So far we have described how a mobile can determine its distance from agiven reflector in addition to the AOA at that reflector. In order forthe mobile to correctly determine its position, it should determinewhich reflector a given signal reflection is coming from, in case thereare more than one reflector in the vicinity. It also should know theposition and orientation of that reflector along with the frequency vs.AOA arrival characteristics for each reflector and the fractionalreflected power ratio for the first reflection of a reflector. Thefrequency vs. AOA arrival characteristics, the approximate embeddeddelay and the fractional reflected power fraction for the firstreflection, could be determined either by design or by measurementduring the manufacturing process of such reflector and a uniquereflector type number could be associated with each distinctcharacteristics associated with a given reflector variant. Duringinstallation, a lookup table is populated that contains a reflector IDnumber, a frequency characteristics number and entries for physicallocation and orientation of each installed reflector. In addition, anentry is also included in the table that gives the approximate built indelay, if any, each reflector employs. This information is made toreside on some server in the internet cloud, whether local or global,and made accessible to all devices that are allowed to perform aposition location fix within some vicinity. The reflectors' location andorientations could be given relative to an indoor landmark or givenrelative to a standard geographic surveying and positioning datum. Whilean accurate tabulation of the reflector delays is not strictly neededsince the mobile can determine it from the fractional reflection methoddescribed above, it can nevertheless be provided to reduce the timeduring which a mobile is listening to a possible reflection.

A mobile can also determine which reflector a given reflection could becoming from. Various techniques can be used for reflectoridentification. One embodiment is based on characteristics (e.g., delayand/or filtering) that a given delay element, such as a bulk acousticwave device (BAW), within a reflector exhibits. These built in reflectordelays are much larger than the longest free air round trip delay andhence can be easily distinguishable by having each reflector within aclose proximity exhibit a different built in delay from those of itsneighbors. Also, these delay elements can have differing frequencyfiltering characteristics thereby aiding in correct reflector-reflectionassignments.

Another embodiment uses a known previous or approximate location of themobile to rule out erroneous mappings of reflection to reflectors. Forexample, in spaces where there are few reflectors that are sparselylocated, it is possible on some occasions for a mobile to receive areflection from only one reflector. Using such a reflection, the mobileis able to determine the distance from and AOA at a reflector. Usingthis information, the mobile calculates a possible mobile location basedon this distant and AOA for each reflector in the extended vicinity,including reflectors residing in above and below floors and withinadjacent rooms. Having an approximate idea about its position from aprevious and recent position fix or by using any of the other inaccuratemethods currently employed for indoor positioning, the mobile can withhigh probability quickly rule out all of the erroneous positions andhence can pick the right position and determine the correct reflector itis detected using at that instant. A refinement of this embodimentinvolves throwing away the erroneous positions based on physicallyimpossible location such as being inside a wall or below a floor oroutside a window just to name a few examples. It should be noted that inaddition to the calculated distance, the fact that embodiments of thesystems described herein also determine AOA at the reflector greatlyimproves the odds of detecting false locations and identifying thecorrect location associated with the correctly identified reflector.This information can be used for subsequent fixes as aiding information.Kalman filtering techniques could be used to further make the determinedposition more robust to noisy measurement.

Another embodiment to determine which reflector a reflection is comingfrom involves using the radar signal reflected power. Given theretro-directive nature of the reflectors, the method of fractional powerreflection outlined above for embedded delay quantifying, and the factthat conditions of line of sight are expected to be prevalent foraccurate positions, it becomes possible to calculate with some accuracythe expected power level that would have returned from each givenreflector had the reflection come from that given reflector. This givesan additional indication if the reflection is line-of-sight or is comingthrough walls, ceilings or floors. It could also indicate if thereflection is bouncing off of another object in between the reflectorand the mobile device as well. All of this information aids in correctlyassigning a detected reflection to the respective reflector correctlydetermine the mobile position. It also determines the degree ofconfidence the last location has been calculated with. Furthermore wecan attempt to distinguish reflectors in the same vicinity by havingeach reflector differ from others by the fractional reflectioncoefficient of each reflector which was used to allow for accuratemeasurement of the built in reflector delay. By measuring the powerdifference between subsequent reflections, we can determine an estimateof this coefficient for each reflected signal and we then can associateit with a specific reflector using the information in the public table.

In an alternate embodiment, we can design the reflectors in the vicinityto have differing frequency vs. main lobe sweep angle characteristics.For example, in an embodiment, one reflector could have a 5° of mainlobe sweep per one MHz of frequency sweep while another can have 7° ofsweep per MHz of frequency sweep. Once a mobile has detected a maximumreflection frequency for a given reflection, it then sweeps thisfrequency and monitors how fast the reflected power drops off. Becausethe mobile already knows the distance to the reflector for a givenreflection, the mobile can predict how fast the reflected power shoulddrop off as the excitation frequency moves off of the optimal frequency.By comparing this result for each of the reflections, and knowing thesweep characteristics for reflectors in the vicinity from the publishedinstallation data, the mobile is able to correctly assign each reflectedsignal to the correct corresponding reflector and hence the mobiledevice physical location can accurately be determined. This idea ofreflectors having differing frequency sweep characteristics could beexpanded to other frequency characteristics such as having eachreflector respond to different frequency sub-bands within a given band.In one embodiment, the first reflector does a full sweep of thedirection of the main lobe across the whole space using a frequency spanof f₀ to f₁. Another reflector uses f₁ to f₂ to do the same. This allowsunambiguous determination of which reflection belongs to whichreflector.

All of the above embodiments that sought to determine from whichreflector in the publicly published reflector table a given reflectionis coming from have assumed that the mobile receives reflections fromone and only one reflector. If however the mobile is able to receivereflections from two or more reflectors, determining which reflectioncame from which reflector becomes simpler. In most cases if we were toerroneously swap the allocation of the reflections to the reflectors,the new computed position would necessitate very different maximumreflection frequencies for the AOAs at the reflectors for convergencethan what was measured thus detecting the error. We then choose thecorrect position with the correct reflector-reflection assignment. Insome cases, specifically where the mobile is equidistant from thereflectors and at the same time far away from them, it becomes morechallenging to guess at the correct reflection-reflector assignment.Fortunately, under such assignment, both of the computed positions, theones with correct and incorrect reflector-reflection assignment, areclose to each other and the position error due to the possible erroneousassignment is small. Nonetheless, it is expected that under suchsituation, and being far from two reflectors, that another reflectorwould be nearby and would give a more accurate position and resolves theerror. We can reduce such occurrences of bad geometry by payingattention to reflector positioning. Also, we can mix horizontal sweepingreflectors with vertical sweeping reflectors to further reduce badgeometry occurrences.

In an embodiment, the system uses some or all of the techniques andmethods outline above to correctly assign reflection to their respectivereflectors thus resulting in correct computation of the mobile'sposition.

Systems disclosed herein can operate at a number of RF bands includingthe available licensed or unlicensed bands. The higher the frequency,the smaller the antenna structure needs to be physically to achieve thesame directivity and antenna structure gain response. Also higher bandsprovide higher spatial resolution of position. On the other hand, higherfrequencies result in more expensive Tx and Rx electronics and higherpower consumption in the mobile device. Higher frequencies when coupledwith carrier phase ranging techniques leads to very accurate positiondetermination limited only by available Signal-to-Noise ratio and by theused bandwidth as discussed below.

The structure of the radar signal that can be used by the mobile alongwith the aforementioned reflectors in order to arrive at a good estimateof the mobile distance to a reflector with dynamically varying degreesaccuracies as needed by the applications requirement at a given instantis now described. The theoretical limit on accuracy within the rangingapplication is given by the Cramer-Rao lower bound (CRLB) (see, e.g.,“Fundamentals of Radar Signal Processing” by Mark A. Richards, 2014 and“RF Ranging for Location Awareness”, UC Berkeley technical report, byLanzisera and Pister, 2009).

For ranging applications the CRLB is given by the formula

$\begin{matrix}{\sigma_{r}^{2} \geq {\frac{c^{2}}{\left( {2\pi\; B} \right)^{2}{E_{s}/N_{0}}}\left( {1 + \frac{1}{E_{s}/N_{0}}} \right)}} & (1)\end{matrix}$

-   -   where σ_(r) is the range measurement variance, c is the speed of        light, B is the signal bandwidth and E_(s)/N₀ is a measure of        Signal-to-Noise (SNR) ratio at the receiver. Equation (1)        simplifies to

$\begin{matrix}{\sigma_{r}^{2} \geq \frac{c^{2}}{\left( {2\pi\; B} \right)^{2}{E_{s}/N_{0}}}} & (2)\end{matrix}$

-   -   for typical Signal-to-Noise ratios expected to be used in indoor        ranging.

We can see from the formula that we are able to reduce σ and henceincrease range accuracy and resolution, by either increasing the signalbandwidth (BW) or by increasing the signal SNR for our signal or both.For RF signals, increasing bandwidth can be done in a number of ways.All of them require that the RF signal be modulated in a number ofdifferent ways. Early continuous wave radars (CW) used single frequencytone for the radar signal. Typically that tone was switched on and offto increase maximum range. In addition, the time when the wave was onwas made as short as possible to increase BW and hence increase rangeresolution. This inefficient way of increasing BW results in very highpeak-to-average ratios in the transmitting power amplifier and also doesnot easily allow for multiple users within the same vicinity.

In one embodiment, the mobile uses the method of Frequency Modulation,both linear and non-linear sweeps, with or without pulse windowing, toincrease the signal BW. Given the same SNR, frequency modulation canincrease range resolution by 1/BW in time and 2c/BW in distance. Linearfrequency modulation (LFM) is a technology used in some radar systems.For the reflectors disclosed herein, note that changing the frequencychanges the angle of directivity at the reflectors. For this reason, theextent of frequency sweep (BW) is limited in this embodiment to BWduring which the reflector's directivity changes little. Hence in thisembodiment, changing the center frequency of the signal results indifferent reflector directivity angles. But changing the frequencyaround the center frequency by a smaller BW such as in the case of LFMresults in minor directivity changes to insure that retro-directivity ispreserved during LFM sweeps. LFM frequency modulation is used in anembodiment. But many other frequency modulations instead of LFM can alsobe used including non-linear frequency modulation, windowed frequencymodulation, gated LFM, up sweep and down sweep LFM, as well as steppedchirp waveform (see, e.g., “Fundamentals of Radar Signal Processing”).

In one embodiment, co-located mobile devices using frequency modulationcan choose different sweep rates, differing gating ON/OFF times, anddifferent center frequencies as dictated by the reflectors todistinguish their radar signals from other mobile devices nearby.

In an embodiment, a phase modulated signal is used. Example phasemodulation includes BPSK or QPSK types. The carrier is modulated usingdirect sequence spread spectrum (DSSS) to increase the signal bandwidth(BW). DSSS serves two purposes in this embodiment. It increases the BWand hence improves range measurement accuracy according to the CRLB. Italso provides multiple-access capability. Multiple-access refers to theability to support numerous mobile devices within the same vicinity suchas the case in shopping centers and sports events. This multiple accesscould be achieved by using different pseudo random number sequences(PRN) as is currently employed in GPS to distinguish among differentsatellite signals or by using the same PRN but separating users by PRNoffsets larger than any expected reflection delay, as is used today inCDMA communications systems to separate different mobiles and cell siteson the same frequency (see, e.g., “CDMA RF Systems Engineering”, SamuelC. Yang, 1998).

In one embodiment, instead of using pseudorandom number (PRN) sequences,a mobile uses truly random number (TRN) sequences. PRN sequences arecalled pseudorandom because they are actually not random but aregenerated according to a mathematical relation designed to produce asequence that mimics as much as possible a true random sequence.

Being truly random, TRNs by definition have infinite cycle length. Theycan be generated on the fly within a mobile by digitizing a naturallyoccurring random phenomenon, such as amplified thermal noise, or diodereverse biased avalanche noise. Such TRNs have theoretically zero crosscorrelation with other TRNs and hence signals coded using them reallylooks like higher thermal noise to all other listeners or to any copy ofitself shifted in time by one or more chip period. Using TRNs ispossible since both the Tx processing and Rx processing is performed inthe same device given the passive and linear nature of the reflectors.With Tx and Rx happening on the same device, TRN usage is done bykeeping a temporary copy of the true random sequence values used togenerate the TX signal until these are value are used for correlating,or de-spreading, the reflections at Rx. All other previously proposedindoor solutions where the sender of the ranging signal is physicallyseparate from the receiver, would not be able to use a TRN in real time.This removes any limit on integration time and allows the mobile tochoose whatever integration time it needs to attain the accuracy itdesires at a given instant.

Even though the mobile clock need not be synchronized to absolute timewith external sources for effective signal correlation because both Rxand Tx reside on the same mobile and use the same local oscillator, themobile clock however should be accurate enough to time the reflectionstime of flight at the speed of light to yield the desired rangeaccuracy. In other words, while it is not important for the mobile clockto keep absolute global time, the local mobile clock should measure thetime from Tx to Rx reflection accurately. Insuring that a clock keepsaccurate differential time, Rx_time−Tx_Time, is a much simpler problemthan to insure that the local mobile clock accurately keeps absolutetime. In an embodiment, the mobile is made to keep accurate differentialtime by locking the local oscillator to any present RF signal availablein the environment known to be derived from accurate reference and thatthis source is not moving so as not shift its apparent frequency due toDoppler. For indoor environments the user movement is slow enough tomake neglecting Doppler due to user movement acceptable except for themost stringent applications. Examples of external stationary referencesources with very accurate frequency references include broadcast radioor TV signals or any cellular network, including asynchronous networks.In fact all modern cellular mobile phones, such as CDMA and LTE, alreadydo that to enable proper communications between the mobile and a cellsite. This locking of the local oscillator to a cellular network signalhave also been used to allow for longer GPS signal correlation times insome outdoor position location implementations. Once the localoscillator is locked to a cell site pilot signal for example, the localoscillator period is adjusted to be equal to that of the GPSconstellation detected at the cell site. In effect this operation allowsa relatively inexpensive local oscillator to maintain “stop watch”accuracy as accurate as GPS time, thus achieving very high accuracyrange measurement. Please note that even after oscillator lock, themobile device's absolute time may differ from absolute time by anunknown mobile to cell site propagation delay. Fortunately, absolutetime according to an external datum is not needed due to the passivenature of the reflectors. This technique works indoors because of theubiquitous cellular coverage in today's indoor environment. Locking theoscillator to an external reference may be performed when very highpositional accuracy is desired. In other applications, a crystal-basedlocal reference oscillator has sufficient accuracy.

It is important to note here that in an embodiment the built-in delaywithin a given reflector when phase modulation is used has to be largerthan a single PRN or TRN chip period. This is because it is expected inindoor environments that a distance traveled during a single chip periodat a typical chipping rate is much longer than the measured rangesencountered. For example at a 10 MHz chipping rate, the chip length indistance is about 30 meters in free air. That means that it is highlylikely that a signal would exit the mobile TX, travel to a reflector andback to the mobile Rx all within a single code chip. This means that themobile needs the reflectors built in delay to be greater than one chipto distinguish between a Tx signal and its echo Rx signal from the samemobile. Furthermore, every reflector should have a delay different fromevery other reflector by a minimum of one code chip period, 100 ns for10 MHz chipping rate for example, in order to separate reflections fromvarious reflectors in the vicinity. This can easily be accomplished withsimple BAW filters in one embodiment. Also, in implementations where thereflectors employ partial reflections, reflector delays should differfrom one another by more than one period to allow the multiplefractional power reflections to die down from a given reflector toprevent secondary reflections from a near reflector matching the PRNdelay of a far reflector with longer built in delay.

Systems disclosed herein can be insensitive to the near/far problemexperienced by pseudolites and beacons for indoor position location.This is because most of the time reflection from different reflectorsoccur at different center frequency due to the reflector frequency anglesweeping feature and hence can be isolated based on frequency bases. Inaddition the ability to use long PRNs as well as the theoreticallyinfinite cycle TRNs allows for better separation of near/far ownreflection signals due to low cross correlation values for such codes.Long PRNs or TRNs are made feasible by the fact that the same localoscillator is used for both spreading and dispreading the signal up tothe limit of the oscillator phase noise. Furthermore, under verycongested conditions, a central network radio link manager can arbitrateamong mobiles to insure that intolerable cross interference does notoccur. This manager could reside somewhere in the internet cloud. Thisis possible because the reflectors are passive, as opposed to radiatingbeacons or pseudolites, and hence only radiate signals that come frommobiles in the vicinity. The radio link manager can allow only amanageable number of mobiles simultaneous-access to use the reflectorsin the vicinity to insure useful position location determination for allat the desired or achievable level of accuracy. System can be veryinsensitive to the near/far problem of other systems because the mobileis able to determine its location using one near reflector. Detectedadditional far reflectors improves accuracy but is not needed for useful3D position determination. Therefore, in the unlikely event that amobile receives reflections from two reflectors at the same centerfrequency with one reflection having much higher power than the other,the mobile possibly seeing one reflector still can usefully calculateits position and in a short time, a little movement changes the antennasweep center frequency allowing the mobile to detect the other reflectorand hence to refine the previous position determination.

In an embodiment, the mobile receiving a very strong reflection for anear reflector that masks other reflectors can reduce the reflectedpower from this near reflector by detuning the transmitted frequencyfrom the optimal that results in maximum reflection in the direction ofthe mobile. This can seriously degrade the reflected power from the nearreflector allowing the mobile to detect signals from far reflectors withmuch lower reflected power. The mobile can also perform separate powercontrol on each center frequency exciting a different reflector toreduce the needed Rx dynamic range due to near/far phenomenon.

At locations that are expected to have single users or low number ofsparse location determining users, such as within private residences,the primary purpose for phase modulation of the radar signal is solelyto increase the signal bandwidth BW so as to result in finer rangeresolution between a mobile and a reflector. Under such scenarios, andin one embodiment, the mobile can employ relatively short PRN codes suchas baker codes or products of shifted baker codes or any number of othercodes (see, e.g., section 4.10 in “Fundamentals of Radar SignalProcessing”). Here the desired BW increase is a consequence of the chiprate and not the PRN sequence length. Hence, even though these sequencesare short, one can still attain the BW increase, and hence rangeresolution improvements, by a chipping rate of approximately 1/BW. Giventhe low number of users, SNR improvement to increase accuracy can bedone by increasing output power since the probability of adjacent usersis low. Still, even for these sparse environments, longer codes canstill be used to improve SNR.

In an embodiment, at locations where one expects numerous mobile devicesto be attempting to determine their location within close proximity ofeach other, a longer PRN sequences or even TRN sequences are used. Withlonger sequences two system architectures present themselves. The firstarchitecture does not allow for simultaneous TX and RX to occur withinthe same mobile (Half Duplex). The second does allow for simultaneous TXand Rx at the same time (Full Duplex). In radar systems Half Duplex havebeen traditionally favored. This is because in radar systems thereceived signal is a delayed copy of the transmitted signal. Hencetypical channel separation between Tx and Rx using simple bandseparation may not be practical. So for the majority of radar systems todate, the Tx and Rx activities are time interleaved to that the Rx doesnot get overwhelmed by the Tx signal leaking back into the Rx path.

In one embodiment, a mobile device uses long PRN or TRN sequences inmodulating the phase of the transmitted radar signal and timeinterleaves the Tx and Rx activities. To get the full benefit of a PRNsequence, one should send at least one full PRN sequence length. Duringwhich time the Rx side is masked off. In order for the Tx and Rx not tooverlap, either of two alternatives are employed. The first is that thebuilt in delays employed in the reflectors are made long enough toaccommodate one full length of a PRN sequence. In other words, noreflection is sent back while a full length of the PRN is transmitted,after which Tx is turned off and Rx is enabled just before receiving thereflections. Another alternative is to time interleave Tx and Rxactivities within a single PRN sequence length. For example, Tx isturned on and transmits a portion of the PRN sequence length then turnsoff Tx enabling Rx to listen to echoes of reflections. After a period ofexpected reflections, Tx is turned on to transmit another portion of thePRN sequence while the Rx is disabled and so one. This interleavingcontinues until the PRN total sequence length is transmitted. Thisinterleaving has the result of keeping the built in reflector delay frombeing large but at the same time preserves the advantages of long PRNsequences. In both alternatives the period between sending the firstchip and receiving the last chip in the sequence is theoretically thesame. However in the case of the portion interleaving, some guard timebetween Tx and Rx switching should be accounted for in addition to thevariable delays of the reflectors.

In another embodiment, both Tx and Rx occur simultaneously (FullDuplex). In radar systems, the Tx signal splashing the Rx path is knownwith great accuracy since it is constructed within the same mobile.Under such scenarios, active noise cancellation techniques can be usedon the Rx signal path to subtract out the coupled and undesirable Txsplashing. Circulators, cross antenna polarization and active noisecancellation may, for example, be used. This gives sufficient dynamicrange to useful indoor position location. The fact that all reflectorshave a minimum of one code chip delay means that close reflections fromthe environment could be attenuated by the dispreading Rx operation.Also, the use of long PRN or TRN codes allows for higher processinggains to ameliorate this isolation requirement and is further madepossible by the negligible cross correlation due to negligible relativeDoppler shift due to the slow relative motion between the mobile and thereflectors. Please note that due to the retro-directive property of thereflectors, the radar reflection is much stronger than in typical radarapplications and hence lower dynamic range is required leading to lessrequired Tx/Rx isolation.

In addition to the various embodiments of modulating the signal both toincrease BW and/or allow various multiple-access operation, we can alsoimprove range resolution by increasing the SNR ratio as stated in theCRLB equation. Increasing SNR is done in two ways. In one embodiment theSNR is increased by increasing the average transmitted power for theradar signal from the mobile. This can be done until the FCC limit formaximum radiated power from a mobile device is reached for a given bandor when the battery power consumption becomes prohibitive. In anotherembodiment, the Rx correlation time is increased thus increasing theprocessing gain of the Rx and improving the equivalent SNR. Increasedcorrelation time requires tight synchronization between Tx and Rx masterclock for frequency and period accuracy. Fortunately, systems disclosedherein use passive reflectors and hence both the Tx and Rx signals usethe same clock within the same mobile. Under such circumstances, thecorrelation time is only limited by how much the local clock driftsduring the time it takes for a reflection round trip delay, includingthe reflector built-in delay, typically in the order of microseconds,and hence the correlation time could be infinite even for inexpensiveoscillators. The longer the coherent correlation time, the higher theprocessing gain and hence the higher the achieved SNR. An example ofthis is what happens in GPS where the received signals are typically −25dB relative to thermal noise floor yet the receiver is able to detectand process the signal due to processing gain using long correlationtimes.

In GPS, the emitted DSSS phase modulated carrier is not filtered andhence the spectrum approximates a SINC function that extends infinitelyto the left and to the right of the carrier frequency. GPS is able to dothis since the received signal power at sea level is about 25 dB belownatural thermal noise level. For terrestrial communications, the powerlevel is higher and the out of band emissions should be filtered.Therefore a typical terrestrial DSSS transmission has a flat spectrumpower level within the transmitted BW and very low emissions outside ofthat BW. Given the frequency sweeping nature of the reflectors we wouldexpect that the reflected signal has a spectrum that is not flat withinthe transmitted BW. This is because the gain of a reflector drops as thefrequency gets away from the frequency that gives the maximum reflectionin the direction of the mobile. If we assume that the mobile istransmitting the DSSS where the center frequency of the transmittedsignal is exactly the frequency needed for maximum reflection in thedirection of the mobile, then one would expect that the spectrum of thereflected signal would not be flat but would fall off in power equallyas we get away from the center to either side. If the transmitted signalhad a center frequency that was slightly higher or lower than thefrequency needed for optimal reflection in the direction of the mobilethen that peak power would be offset from the center of BW to one sideor the other. This provides an indication to the mobile that it shouldadjust its center frequency, and in which direction, in order to trackthe frequency that is needed for maximum reflection for a givenreflector in the direction of the mobile and hence a symmetric reflectedspectral power around the adjusted center frequency. Which also providean indication for the AOA without needing to continuously sweep thefrequency.

In one embodiment, the mobile is flexible enough to increase or decreasethe effort it utilizes to achieve various levels of accuracy. As we havestated previously, increased accuracy demands increasing either or allof signal BW, transmitted power level and coherent Rx correlation time.Increased BW results in some of the signal experiencing lower reflectorantenna gain in the direction of the mobile with some of the signalbeing reflected away from the mobile and hence increasing interferenceto other mobiles in the vicinity. Increasing transmitted power reducesbattery life and also increases the interference to others. Finallyincreasing correlation time also decreases battery life and increasesbackground noise to others due to longer transmitted signal duration anddemand longer time-to-fix periods. So it becomes useful for the mobileto dial down the effort it expends whenever high accuracy is not needed.In an embodiment, the mobile performs active transmitted power controland dynamic BW selection and correlation time selection to achieve therequired range accuracy at the time.

In one embodiment, the mobile makes use of carrier phase measurement forrelative movement location updates. Until now we have only consideredposition determination based on code phase methods. This limits positionaccuracy to a fraction of a PRN chip length the fineness of whichdepends on BW and SNR as outlined above. However, carrier phase methodscan achieve position accuracies to within a fraction of a carrier centerfrequency wavelength. This is orders of magnitude finer resolution thancode phase techniques. In any coherent receiver, the carrier phase isknown but the integer number of wavelength that the signal traveled toand from the reflector is unknown. This is called the carrier integerambiguity (N). If we know N and we also already know the residual phaseof the carrier at Rx and Tx, we can calculate the range to a smallfraction of a carrier wavelength absolute accuracy. Under favorableconditions, the user BW and available SNR are sufficient to narrow thevariance of code phase determined range to below 1/2 carrier wavelength.Theoretically this is sufficient to determine N for carrier phasecalculation and enable very accurate positioning. Practically one needsto get the code phase position variance down to less than ¼ carrierwavelength for reliable N determination. Once N is determined for one ormore reflectors, we can relax the effort expended to determine N andkeep an accurate knowledge of N for each reflector by continuouslytracking the carrier phase of the received signal and incrementing ordecrementing N with every complete phase wrap around in eitherdirection. In other words, the mobile expends great effort to increaseBW possibly by increasing chipping rate and or increasing SNR by higherTx power or longer Rx correlation times to determine N, then switches tocontinuous carrier phase tracking to maintain very accurate positioningand relaxes by reducing BW and or SNR it needed for determining N. If atany time carrier tracking is lost due to environmental conditions, themobile reverts back to rely more heavily on Code Phase methods forlocation determination and N determination again.

In embodiments that use carrier phase ranging techniques, the mobilecompensates for the carrier phase shift that results when the main gainlobe of a frequency swept reflector swings as a function of excitationfrequency. Without compensating for this effect, the full benefit ofcarrier phase ranging techniques may not be realized. Fortunately, themobile knows the excitation frequency and is aware from the publishedreflector data of the angle of the main lobe and hence can estimate andcompensate for this phase shift that is typical of frequency sweptphased arrays.

In cases where the environment does not allow for reliable determinationof N from code phase methods given the permissible BW and or SNRs, wecan track the relative movement of the mobile by tracking relative Nincrements/decrements, through full period phase changes, and residualcarrier phase changes from a previous positions enabling very accuraterelative motion measurement. For a stationary mobile with stationaryreflectors, this method admittedly does not yield additional informationto determine absolute position. However, if the mobile is moving and isreceiving reflections from multiple reflectors it becomes possible aftersome time, while keeping the tracking history, to determine this carrierphase N ambiguity and transition to the high accuracy absolute trackingmode especially if the geometry among reflectors and the mobile changesappreciably as is used by the Integrity Beacon Landing System usingPseudolites and GPS satellite signals (see, e.g., “Global PositioningSystem: Theory and Applications Volume II”, by Parkinson and Spilker,1996, Chapter 15 and as described in “GPS Pseudolites: Theory, Designand Applications”, H. Steward Cobb, PhD Thesis, Stanford University,1997). This method works if three or more reflectors are visible to themobile.

In previous embodiment, the mobile tracks the carrier phase to maintainaccurate positioning. This may not be possible due to multiple accessrestrictions in crowded environments. For such cases, a system employstechniques that can resolve N very quickly. Because N can be resolvedwith minimum time, no tracking is needed since it is just as efficientto determine N at every fix rather than determine N once and maintaincarrier tracking after that. Higher chipping rates and multiplefrequencies can be used to determine N quickly with minimum time andmovement to enable carrier phase range accuracies even in crowdedenvironments such a sports stadium.

In one embodiment, multiple frequencies are used along with higherchipping rate to produce enough information to resolve N on the fly andenable carrier phase ranging. Those unfamiliar with this art may, forexample, refer to Stone, Jonathan et al, “Carrier Phase IntegerAmbiguity Resolution Using Dual Frequency Pseudolites”, 11^(th)International Tech, Meeting of the Satellite Division of ION, Sep. 1998,pp 961-968 and Forssen, B. “Comparison of wide-laning and tone-ranging”,Electronic Letters Aug. 28, 1997, vol. 33, No. 18, pp. 1525-1526. Beingreflector based, the systems disclosed herein do not need to correct forτ and therefore can resolve carrier phase ambiguity for each reflectorindependently from the position calculation process. In addition, in oneembodiment, the excitation frequency is sometimes dithered slightly toallow for better probabilistic determination of N under circumstancesthat fixed frequencies can allow for.

Multi-Frequency Reflectors are built out of multiple collocated sweepingreflectors that respond to different RF bands. One reflector is designedto respond to sweeps from f₀ to f₁, while another is made to respond toa band from f₂ to f₃. If these reflectors are made to sweep along thesame plane, the two sub-bands can only be partially overlapping ordisjoint to insure that at any given excitation frequency, the directionof main gain is different from one reflector to the next and hence twofrequencies are used for any reflector to mobile angle. The antennaports of these collocated reflectors are connected to the same delayelement by an RF combiner-splitter and hence have identical built-indelays. A mobile can now excite this multiple reflector using multiplefrequencies and hence is able to resolve the carrier phase integerambiguity N using one compound reflector and a short measurement time.This leads to increase system capacity in congested environment whilestill enabling carrier phase level accuracies.

In one embodiment, a system resolves the carrier integer ambiguity usingfrequency sweeping. A transmitted radar signal reflecting off of one ofthe reflectors, as, experiences a delay equal to the round trip free airdelay in addition to the built in reflector delay. This delay translatesto a number of integer carrier cycles and a residual carrier phase. Bycomparing the Tx phase to the Rx phase we can determine this residualphase. As we change the excitation frequency, the integer number ofcycles as well as the residual phase both change. If we track thischange as a function of swept excitation frequency, we would be able todetermine the integer number of cycles of the reflected signal. Giventhe typical built in delays of the reflectors, on the order ofmicroseconds, and the typical excitation frequencies, on the order ofGHz, the expected number of round trip cycles would be in the hundredsor thousands of cycles. That means for a 0.1% frequency change in Tx, afull cycle or more is either added or subtracted from the round trippath. Mobiles can generate very accurate center frequencies. By sweepingthe frequency while tracking the change in Rx carrier phase, includingcounting full cycle phase accumulation, we can determine the ratio of Rxphase change to Tx period fractional change which gives N. Havingdetermined N, we now can calculate the range with high degree ofaccuracy, after subtracting the built in reflector delay.

A mobile may use only one reflector to determine 3D position, due to theangle provided by each reflector. For example each reflector can have avertical sweeping reflector in addition to collocated horizontalsweeping reflector to give both azimuth and elevation angles. Theaccuracy of these angles depends on sweeping sensitivity and SNR. Ifhowever multiple reflectors are used, along with carrier phasetechniques, the accuracy of such a system is not fundamentally limitedand any accuracy can be achieved if enough BW, SNR, averaging isavailable at a given environment.

In an embodiment, the publicly accessible server, in addition tocontaining information about each reflector and also managing theavailable bandwidth in the vicinity to allow for fair multiple accesscan also exchange position location aiding information that can helpeach mobile determine its location accurately with less effort by usingresults published by other mobiles. An example of this aiding is themeasured value of reflector built in delays. In one embodiment, eachmobile can publish its location and also act as a reflector bypublishing a specific delay it shall use and its own PRN sequenceformula. A mobile would then know the PRN of another mobile and woulddetect the other's radar Tx signal, it would then delay it by thepublished amount, then retransmit it back coherently using the same PRNsequence of the other mobile. This makes this cooperative mobile act asanother reflector, providing additional piece of ranging information butalbeit without the angle information that a true passive frequency sweptreflector could.

FIG. 1 is a functional block diagram of an indoor positioning systemaccording to a presently disclosed embodiment. The system includes amobile 185 whose location will be determined. The mobile 185 may be anyelectronic device (e.g., a smartphone). The system also includes aserver 194. The mobile 185 communicates with the server 194 via anetwork 192. The network 192 may be, for example, a cellular network ora Wi-Fi network. The system also includes a position reflector 181. Themobile 185 transmits radar signals 182 and senses echoes from theposition reflector 181 for use in determining the position of the mobile185. For ease of illustration, the system of FIG. 1 includes only oneposition reflector; a location may include multiple position reflectors.Although the positioning system is particular suited for indoor use, itis not so limited.

Before transmitting the radar signal, the mobile 185 may determines itsapproximate (coarse) location or neighborhood. For example, the mobile185 may use currently available positioning methods or may collectinformation from the environment that can be used for this neighborhooddetermination. One example of this coarse location determination is tosave the last known outdoor GPS location which can be used to determinewhich building or collection of buildings the mobile 185 might be in.Alternatively or additionally, the mobile 185 may narrow its approximateposition through detection of available Wi-Fi networks that have knownlocations. This approximate position could be a position with tens orhundreds of meters of uncertainty. Using this approximate position, themobile 185 connects to the server 194. The server 194 may becommunicated with using a known and published Internet protocol (IP)address. The connection from mobile 185 to server 194 may be routedthrough the Internet. The server may be a dedicated separate server ormay be a shared server with capacity rented as needed from a serverfarm.

Having established a connection to the server 194, the mobile 185 sends,to the server 194, its approximate location or any environmentalinformation through which the server 194 can approximate the coarselocation of the mobile 185. The server 194 can then send to the mobile185 a list of reflector data about reflectors that may be detectablewithin the approximate location of the mobile 185.

The mobile 185 then transmits a radar signal and listens for thereflections from reflectors on the server provided list of reflectordata. As described further below, the mobile 185 is able to determinefrom which reflector, using the list of reflector data, the detectedreflection came from, the range to the reflector, the angle-of-arrival(AOA) of the radar signal at the reflector, or both the range and AOA.Using the reflector data (e.g., information about the location,orientation, and frequency and delay characteristics of the reflector),the mobile 185 can determine its location accurately.

Once an initial accurate position is determined, the mobile 185 maylimit its search for reflections to only those reflections that can comefrom reflectors that are located in the immediate vicinity. This mayimprove the position determination time-to-fix and can improve accuracysince the mobile 185 can now expend more effort, e.g. longer correlationtime per hypothesis, towards detecting more than one reflector from amuch reduced number of reflectors that are in the immediate vicinity andhence improve accuracy. Having determined its location accurately, themobile can then track its differential position changes from thisoriginal fix very accurately, for example, by tracking the carrier phasecycle accumulation associated with each reflector. This may only donefor the most demanding of applications since it may use more processingand more power consumption due to continuous tracking.

FIG. 2 illustrates an example location positioning according to apresently disclosed embodiment. FIG. 2 shows an example indoor room withwalls 200 and a floor. A mobile 285 is equipped with position locationtechnology as disclosed herein is present in the room. In the example ofFIG. 2, a position reflector 281 is affixed to a corner of the room.When the mobile 285 wants to determine its position, it can emit aspecially encoded radar signal. This signal may radiateomnidirectionally from the mobile 285. Some of this signal is radiatedin the direction of the position reflector 281 along the path 203. Theposition reflector 281 in the example of FIG. 2 includes a frequencyscanning antenna with angle of maximum gain, both in reception andtransmission, that is a function of the center frequency of the incidentsignal. The mobile 285 scans the radiated signal in frequency untilmaximum reflected power is detected. With the mobile already knowing thelocation, orientation, and frequency scanning characteristics of theposition reflector 281, for example, from server provided information,the mobile is able to determine the AOA of the line of sight vector tothe reflector and hence the azimuth and elevation angles of the vectorbetween the mobile 285 and the reflector position along path 203.

In addition, the mobile 285 measures the round trip delay of thereflected signal and calculates the range distance between the mobile285 and the position reflector 281. Having determined two angles and thedistance between the mobile 285 and the position reflector 281, whoselocation and orientation within the room is known, the mobile 285 isable to determine its location in 3D space by detecting only onereflector.

FIG. 3 illustrates another example location positioning according to apresently disclosed embodiment. The space of FIG. 3 is similar to thespace of FIG. 2. However, the space of FIG. 3 includes two positionreflectors (a first position reflector 381 a and second positionreflector 381 b, collectively position reflectors 381). Here, each ofthe position reflectors 381 could yield two AOA angles and a range toprovide an over-determined position calculation resulting in higheraccuracies. Alternatively, the position reflectors 381 could be asimpler type of reflector that yields only one angle and one rangemeasurement. This still provides an overdetermined position calculationwith only two visible position reflectors. If a mobile is able to detectreflections from three or more position reflectors, the positionreflectors need only provide a single AOA each, only a range measurementeach, or any minimum combination of three measurements for the mobile todetermine its location. Each reflector can still provide two AOAs and arange for more accuracy or robustness, for example, through detection oferroneous location determination.

Reflectors types include three categories: the simple passive reflector,the frequency scanned passive reflector, and the active retro-directivereflector. Regardless of their type, all reflectors delay the receivedradar signal by an internal delay before reflecting the signal back.These intentionally included delays are typically in a range between 0.5microseconds and 3 microseconds. They serve four purposes. The first isthat they delay the signal until all ambient multipath signals andechoes from parasitic reflecting items, such as a large facing mirror,have died. Secondly, the delay increase the time between sending theradar signal and receiving it. This increased time makes it feasible forthe mobile device to alternate between active TX, when the radar signalis being transmitted, and active RX, when the signal echoes are expectedto be received. This eliminates the self-jamming problem because both RXand TX are at the same frequency. Thirdly, the preferred embodiment ofthe mobile circuitry is to emit a radar signal that uses direct sequencespread spectrum modulation (DSSS). In a DSSS receiver, echoes fromdifferent reflectors and the environment can only be distinguished ifthey arrive more than one DSSS chip apart. These inserted delays at thereflectors insure that. Finally, these inserted and differing delaysserve as an identifying mark for the mobile to pair up detectedrefection with their respected reflectors.

Reflectors may also have a property referred to asdouble-delayed-reflection (DDR). If a reflector includes a delay elementof d microseconds, the reflector initially echoes back signals itreceives d microseconds after it receives them. With DDR, the reflectorgenerates two echoes. The first echo is delayed by d microseconds andmay be referred to as the primary reflection. The second echo is lowerin power (e.g., −10 dB relative to the primary reflection) and delayedby 2 d or 3 d microseconds. DDR can facilitate eliminating calibrationrequirements for the reflector delays.

FIG. 4 is a functional block diagram of a position reflector accordingto a presently disclosed embodiment. The position reflector of FIG. 4may be referred to as a simple passive reflector. The position reflectorof FIG. 4 may be used to implement the position reflector 181 of FIG. 1,the position reflector 281 of FIG. 2, or the position reflector 381 ofFIG. 3. The position reflector of FIG. 4 includes an antenna 410. Theantenna 410 connects at an antenna port 411 to one port of a delayelement 425 through a matching network 420. The matching network 420 mayinclude a resistor network, a transformer, or other passive circuitelements. In some embodiments, a matching network may be reduced oromitted, for example, when the impedances to be matched are the same orsimilar. The other port of the delay element 425 connects to ground. RFsignals received by the antenna 410 are delivered through the matchingnetwork 420 to the delay element 425. The signal travels one delaythrough the delay element 425 and shows up at the opposite port where itis reflected back by the short to ground through the delay element 425and then reradiates from the antenna 410. If the delay of the delayelement 425 is t microsecond each way, the reflector has 2t microsecondRx to Tx delay. In addition, through slight detuning (impedancemismatching) of the matching network 420, part of the delayed signalcoming back from the delay element 425 gets reinjected back into thedelay element for an additional round trip. Thus, another reflection isradiated by the antenna at time 4t microseconds. The power level of thisreflection is lower than the initial (primary) reflection (e.g., by 10dB or more depending on detuning of the matching network).

FIG. 5 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment. The position reflector ofFIG. 5 is similar to the position reflector of FIG. 4 with likereferenced elements perform like functions. The position reflector ofFIG. 5 includes a circulator 527 that split the Rx and Tx paths. The Rxpath is fed to one port of the delay element 525 and the other port ofthe delay element 525 feeds the Tx path. For this reflector, the initialreflection comes after t microseconds and the second weaker reflectioncomes after 2t or 3t microseconds. Operating these reflectors in the 2.4GHz or 5.0 GHz bands may provide for convenient component sourcing butsuch reflectors may work at other frequencies. The delay elements may beSAW or BAW delay lines for 2.4 GHz and may be BAW delay lines for higherfrequencies such as 5.0 GHz.

These simple passive reflectors contain no power source therebyproviding convenient installation and service. These reflectors have noregenerative gain stages. Thus, the insertion loss of the delay elementsdirectly affects the reflected signal power level. At gigahertzoperating frequencies, 20 dB insertion loss in the delay element mayoccur for the initial reflection.

The simple reflectors (e.g., as illustrated in FIGS. 4 and 5) provideonly ranging information and no AOA information. To determine theposition of the mobile in 3D space, the mobile can get reflections frommultiple non-planar reflectors within the vicinity.

FIG. 6 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment. The position reflector ofFIG. 6 includes a frequency scanned phased array antenna and may bereferred to as a frequency scanned phased array reflector. The positionreflector of FIG. 6 may be used to implement the position reflector 181of FIG. 1, the position reflector 281 of FIG. 2, or the positionreflector 381 of FIG. 3.

The position reflector of FIG. 6 includes an array of antennas 610 a-f.The antennas may be arranged on a straight line. Increasing the numberof antenna and the linear extent of the reflector increases directionalgain and narrows the 3 dB beam width of the main gain lobe. The antennasare coupled to other circuitry in the reflector through an antenna port611. Each antenna may be physically separated from adjacent antennas bydistance D. The separation may correspond to one-half a wavelength of acenter frequency of signals used with the reflector.

A plurality of transmission paths 612 a-f sequentially couple the arrayof the antennas. The transmission paths may be referred to as delaylines. The signal from the antenna port 611 is routed to the firstantenna 610 a where a power splitter/combiner imparts a fraction of theenergy to the first antenna 610 a and the remaining energy continues onvia the first transmission path 612 a to feed the other antennas. Thesignal feeding a subsequent antenna is delayed by the connectingtransmission path. Each transmission patch may have an electrical lengthL. Finally after feeding all of the antenna, the signal is terminatedwith a termination 622. The termination 622 may have an impedancematching that of the antennas and transmission patch so as to prevent(or reduce) residual power reflecting back to the antennas.

This structure of antennas and transmission paths of the reflector maybe referred to as a frequency scanned array (see, e.g., “Introduction toRadar Systems”, M. Skolnik, New York, McGraw-Hill, 3rd Edition, 2001.Chapter 9). The phase difference between two adjacent antennas Ø equals2πLλ where λ is the wavelength of excitation. Thus, Δλ=2λ₀ (D/L) sin θ₁.Where θ₁ is the angle of maximum reflection offset from a broadsidedirection. The direction of maximum reflection changes as the excitationwavelength λ deviates from λ₀, the center wavelength at which maximumbroadside radiation occurs. For example, if L/D=20, varying theexcitation frequency by ±7% results in a sweep of ±45°. In an embodimentwith sufficient bandwidth available for a 90° scan, this simplestructure may be used for indoor positioning as described herein. Inother embodiments, the available span of frequency for scanning may notbe more than ±1% of center frequency. Under such scenarios, twotechniques may be used. In one embodiment, L is made as large as neededto enable a 90° scan of the main gain lobe with the available BW. Thetransmission paths may be made by appropriate lengths of embeddedtransmission lines. At high frequencies, such in the 24 GHz civilianradar band, the required length L is manageable. However, for lowerfrequencies, such as below 5 GHz bands, a large L/D requires a fairlylong transmission line that is physically large. Such embodiments mayuse, for example, loaded transmission lines, ferromagnetic waveguides,BAW filters, SAW filters, distributed LC delay lines, CRLH meta-materialdelay line structures, ceramic waveguides, and/ormicro-electro-mechanical systems (MEMS) delay lines to keep the sizemanageable. Also, active gain components can be inserted in the delaypaths to reduce the loss within these passive delay lines if a very highL/D factor is desired.

In an alternative embodiment, the 90° scan is divided among N adjoiningsectors. Each sector may have a 90°/N scan capability. Each sector caninclude a frequency scanning reflector that can scan a minimum of a90°/N angle. Making all collocated sectored scanning antennas scan inthe same direction, clockwise for example, insures that at sectorboundaries only one antenna sector would be at maximum gain.Distinguishing among sectors may use, for example, the variousmethodologies to distinguish among reflectors described herein.

The position reflector of FIG. 6 includes a delay element 625 with afirst port coupled to the antenna port 611 via a matching network 620.The delay element may be the same or similar to the delay element 425 ofFIG. 4 (e.g., an RF delay passive device, such as a BAW or a SAWfilter). The other port of the delay element 625 is shorted to ground.Here incident radiation on the reflector is delayed by the round tripdelay within delay element 625 before re-radiating from the reflector.The delay allows other parasitic reflections within the vicinity to diedown before reflecting the signal from the reflector back to the mobile.This can significantly reduce the effects of room multipath and improvethe SNR at the mobile.

In one embodiment, the delay of the delay element 625 is longer than anyfree air round trip delay and nearby reflectors have different delaysallowing the mobile to distinguish among reflections coming fromdifferent reflectors when the using, for example, linear frequencymodulation (LFM), non-linear FM, or windowed (LFM) in encoding the radarsignal.

In one embodiment, the signal radiated from the mobile is modulatedusing direct sequence spread spectrum (DSSS) techniques. This is donefor example by using BPSK or QPSK modulation employing a pseudo randomnumber (PRN) sequence, or a true random sequence (TRN), at a given chiprate. If the delay of delay element 625 for a given reflector differsfrom other reflectors by more than one PRN chip period, then the mobilecan distinguish among multiple reflections coming from differentreflectors.

In one embodiment, the matching network 620 is slightly detuned fromproviding a perfect match between the antenna structure and the delayelement 625. This results in part of the incident signal beingreradiated back after one roundtrip delay through delay element 625 andpart being reflected back into the delay element 625. This results inmultiple reflections being re-radiated by the reflector in response toan incident radar signal. The most powerful reflection occurs aftert_(r1)=2t_(air)+2t_(delay), where t_(air) is the free air time of flightfrom mobile to the reflector and t_(delay) is the one way delay throughdelay element 625. Another weaker reflection is emitted aftert_(r2)=2t_(air)+4t_(delay). From these two equations the mobile canaccurately calculate t_(air) and t_(delay) thus compensating for delaydrift in delay element 625 (for example, due to temperature or aging)and hence accurately determining the range to the reflector.

FIG. 7 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment. The position reflector ofFIG. 7 is similar to the position reflector of FIG. 6 with like elementsperforming like functions. The position reflector of FIG. 7 includes aspreading lens 715. Radiation leaving the antenna structure along thepath 703 passes through the spreading lens 715 where it is diffractedand continues on in along path 704. This has the effect of increasingthe antenna scan angle for a given frequency sweep. This antennastructure may be particularly practical at higher (e.g., >10 GHz)frequencies. This lens has inverse directivity compared, for example, toa collimating lens.

Antenna structures in FIGS. 6 and 7 provide scanning in one directionand hence can provide one AOA. To provide two AOAs for single reflectorposition determination, the indoor positioning system can use two copiesof a reflector structure collocated and mounted orthogonally to eachother to provide the two orthogonal angles. Many variations on thesereflectors are possible, for example, reflectors with non-uniform arraysof elements. In another reflector variation using a reflector antennabased on meta-material. An example of this reflector adds a delayelement to a meta-material CRLH leaky wave passive retro-directivereflector (see, e.g., “Electromagnetic Metamaterials: Transmission LineTheory and Microwave Applications,” 2005, by Christophe Caloz and TatsuoItoh, section 6.4.1). A delay element as described herein is insertedbetween the antenna terminal and a short that would create aretro-directive reflector with the delay simplifying the radarprocessing. Such a reflector is advantageously retro-directive, passive,has one leaky radiating element and thus can use only one delay element.Being made from CRLH (meta-material) can improve on the standardmaterial leaky wave antenna performance through allowing widerreflection angles (e.g., almost 180 degrees).

FIG. 8 is a functional block diagram of another position reflectoraccording to a presently disclosed embodiment. The position reflector ofFIG. 8 may be referred to as a phase conjugation array (PCA) reflector.Such reflectors may be particularly suitable, for example, in commercialinstallations where a power source may be available at the positionreflector. Since only a few reflectors per large space are used, thenumber of power sources is small.

The position reflector of FIG. 8 includes an array of reflector elements815 a-d. Each reflector element includes an antenna 810. The antenna iscoupled at an antenna port 811 to a first port of a delay element 825via a matching network 820. The delay element 825 and the matchingnetwork 820 may be implemented as described for other reflectors.Likewise, the matching network 820 can be detuned to provide doubledelayed reflection. The second port of the delay element is coupled to areflector mixer 829. The reflector mixer 829 reflects a signal that isthe incident signal mixed with a signal form a local oscillator 845. Asillustrated in FIG. 8, the reflector mixer 829 may include a circulatorand a mixer. The circulator has a first terminal coupled to the delayelement 825, a second terminal coupled to a first mixer input and athird terminal coupled to a mixer output. The mixer has a second inputcoupled to the local oscillator and produces the mixer output by mixingthe signals on the first and second inputs. The mixer may be active orpassive.

If the local oscillator frequency is twice the frequency of the incidentwave, the PCA reflects the incident wave retro-directively back towardthe source. This retro-directivity can be reduced the more theexcitation frequency deviates from one-half the local oscillatorfrequency. This frequency sensitivity can be used to distinguish amongclose by reflectors with different center frequencies as well as providea way for the mobile to find out the AOA at the reflector by ditheringthe transmitted frequency around a given reflector's retro-directivecenter frequency. Additionally, the spacing for a PCA is unimportant andhence the structure can conform closely to the supporting walls,ceilings, or floors.

The reflector of FIG. 8 includes a digital phase shifter 840. The phaseshifter 840 modulates the signal from the local oscillator by a datasequence to produce either a 0° or a 180° round trip phase shift. Thedata sequence may be a short repeating code with low data rate (e.g.,orders of magnitude lower rate than the chipping rate of the mobile DSSSsignal). Because the data rate of the data sequence is much slower thanthe chipping rate, the mobile can detect its signal and at the same timereconstruct the transmitted data bits from the reflector. These databits can uniquely identify the respective reflector. The mobile may use,for example, techniques similar to those used in GPS receivers forrecovering the 50 Hz data stream. This technique of signaling a datasequence using a phase shifter can be used for other positionreflectors, for example, the reflector of FIG. 4.

The reflector of FIG. 8 can provide range information but may notprovide AOA information to the mobile. However, such a reflector doesreflect the mobile signal retro-directively and hence increases thenumber of mobiles that can be served at the same time within a vicinity,for example, due to RF space diversity and lower required SNR. This isparticular important for large venues with lots of users, for example,in malls, sports arenas, and the like.

Each PCA-based reflector yields one range measurement. A mobile candetermine its location accurately by detecting reflection from four ormore of these PCA based reflectors. For accurate ranging, the delayelements 825 in the different reflector elements 815 should be tightlymatched. Since the reflector is an active device, digital delay linescan be used, for example, instead of SAW or BAW devices. A digital delayline can include a downconverter to convert the signal into a lowerfrequency, an analog-to-digital converter, a digital delay shiftregister, follow by a digital-to-analog converter, and an upconverter toreturn the signal to its original frequencies. Delay elements realizedthis way are perfectly matched (or nearly so). Alternatively, the pathscan be combined into one path and sent into one SAW or BAW delayelement. After passing through the same SAW or BAW, the signals aresplit again into their own separate paths. The signals may be kept frommixing while passing through the same delay element by using frequencydivision or code division RF techniques. Such an approach uses only onedelay element. Because each path is delayed by the same physical SAW orBAW element, the delays are matched.

In a variation of the reflector of FIG. 8, all of the signals aremultiplexed through one delay element using CDMA technique. The signalfrom each matching circuit 820 is passed through a circulator toseparate the Rx path from the Tx path. The Rx path of each antenna isthen mixed with a PN sequence that is unique for each path. After PNmixing, all of the paths are combined and passed through the same delayelement. Exiting the delay element, the combined signal is split intoone path per antenna. Each path is then mixed again with itscorresponding PN sequence that was used before combining but delayed bythe known delay of the common delay element. The signal is passedthrough a band-pass filter to remove mixing harmonics. Then, each pathis mixed with the local oscillator (after phase shifting when included)frequency (which is at twice the center frequency of the radar signal)to generate their phase conjugate signals. After this mixing, the signalis filtered to remove harmonics and the high frequency product componentand then passed on to the Tx side of the path circulator. In doing theabove, each received signal is conjugated and delayed by the same delayas all the other paths because all paths use the same delay element. Thefull structure is then a retro-directive reflector with a built indelay.

In another embodiment, the reflector is a composite reflector includingfour or more reflectors of the types illustrated and described above,where each is retro-directive and reflections from each aredistinguishable from others within the composite reflector. Thereflectors are arranged not to be along a single linear line in order toprovide both azimuth and elevation angle determination. Phasedifferences among each of the reflectors within a composite reflectorare distinguished by a RAKE receiver and hence the two AOA at thecomposite reflector can be determined.

More than three reflectors are used since the distance betweenneighboring reflectors within a composite reflector is more than onewavelength and hence the integer wavelength ambiguity has to be resolvedfor correct AOA determination. The additional reflectors should not beplaced equidistant in order to generate enough information to resolvethe ambiguity. Center frequency detuning, reflected power, and frequencydithering can also be used to resolve the integer ambiguity amongreflectors within a composite reflector.

FIG. 9 is an isometric diagram of a position reflector according to apresently disclosed embodiment. The position reflector of FIG. 9 may beused, for example, to implement the reflectors of FIGS. 6, 7, and 8. Thereflector includes a printed circuit board 911 that has multipleconducting copper layers in addition to the top and bottom layers. Thetop layer 912 is patterned to contain a number of radiating elements(antennas) 910. These radiating elements could be any geometric shape,for example, based on the desired radiation patterns. The radiatingelements are illustrated as rectangles only to provide a clear example.Layers in between the top and bottom layers can be used for constructingthe branching feeding networks. The bottom layer can have traces tomount passive or active circuitry. Since the radiating elements arerepeated horizontally, the direction of maximum radiation aligns withthe vector 903 that is coincident with plane 902. This imaginary planeintersects the reflector with a horizontal line due to the horizontaldistribution of the radiating elements 910. However, depending on theshape of the radiating elements 910, the plane 902 need not be normal tothe plane of the reflector. Also, the radiating elements need not beidentical. Also note that the phasing between the radiating elementscould be done in a way that results in asymmetric scanning of maximumradiation direction with frequency. For example vector 903 could scanmore to the right than the left.

FIG. 10 is a plan view of a position reflector according to a presentlydisclosed embodiment. The reflector of FIG. 10 may be referred to as asectored reflector. For sectored reflectors, multiple linear reflectorsof FIG. 9, each with a different phasing bias are stacked next to eachother to result in a flat sectored reflector.

FIG. 11 is a plan view of another position reflector according to apresently disclosed embodiment. The reflector of FIG. 11 can yield twoAOAs. The reflector includes two adjacent reflectors. Each could be, forexample, a single linear reflector as shown in FIG. 9 or a sectoredreflector as shown in FIG. 10. A first reflector 1120 is oriented withhorizontal radiation plane, and a second reflector 1121 is rotatedrelative to the first reflector 1120 such that the radiation plane isvertical.

FIG. 12 is a functional block diagram of transmit and receive radarchains according to a presently disclosed embodiment. A transmit chain1205 and a receive chain 1255 may be implemented in a mobile device, forexample, using complementary metal oxide semiconductor (CMOS)system-on-a-chip integrated circuits. The illustrated transmit andreceive chains are exemplary and other suitable arrangements may also beused.

The transmit chain 1205 includes an I and Q modulator. For the transmitchain 1205, digital data samples for the in-phase (TX_I) andquadrature-phase (TX_Q) values are supplied, for example, from a digitalsignal processor (DSP)). The I and Q sample streams could be carrying DCcentered, or low IF centered sampled data. The data is converted toanalog levels using digital-to-analog converters (DACs) 1231, 1232,which may also contain anti-aliasing filters. The sampling clock of theDACs may be synchronized with a local oscillator (LO) to maintaincoherence. The sample clock is at least double the bandwidth of thetransmitted signal to avoid aliasing. A local oscillator (LO) containinga phase locked loop (PLL) 1225 is phase locked to a frequency reference1240 and generates IQ signals to up convert the base band I and Qsignals to the carrier frequency using I and Q RF mixers 1221, 1222. Theup converted I and Q signals are summed and filtered in the summingnetwork 1215. The combined signal then passes through a variable gainamplifier 1213 to adjust the transmitted output power as controlled by apower control algorithm, which may be in a baseband or applicationprocessor. The signal then passes through a band selection filter 1211to remove spurious out of band emissions and is then passed on to atransmit antenna 1210.

For the receive chain 1255, the received signal is first routed from areceive antenna 1260 to a band selective filter 1261 to rejectout-of-band signals and improve overall signal-to-noise ratio (SNR). Thesignal is then fed to a variable gain and variable IP3 low noiseamplifier (LNA) 1263. Coming out of the LNA 1263, the signal is splitusing a power splitter 1265. The split signal is down converted toeither base band or low IF I and Q signal streams using RF mixers 1271,1272. The signals are then filtered and sampled by combined filter andanalog-to-digital converter blocks (ADCs) 1281, 1282. The ADC clocks andreceive LO from a receive PLL LO generator 1275 are both synchronizedwith the frequency reference 1240 and thus with the transmit chainthereby maintaining coherence. The digital samples out of the ADCs exitthe receive chain as in-phase samples (RX_I) and quadrature samples(RX_Q). The I and Q receive digital data streams are sent to a receiveDSP in the mobile for further processing.

The transmit and receive chains of FIG. 12 use separate antennas. Thisembodiment can be used for half duplex and can provide some TX-RXisolation during full duplex operation.

FIG. 13 is a functional block diagram of further transmit and receiveradar chains with an alternative antenna arrangement according to apresently disclosed embodiment. The embodiment illustrated in FIG. 13 issimilar to that of FIG. 12 but the transmit and receive chains share anantenna. The transmit chain 1307 and receive chain 1355 are connected toone antenna 1310 through a transmit/receive (T/R) switch 1307. The T/Rswitch 1307 alternately connects the antenna to either the transmitchain 1305 or the receive chain 1355. This is used where transmittingand reception of the radar signal is done at half duplex. In half duplexoperation, the mobile transmits the radar signal for a short time andthen turns off the transmitter and listens for reflections. An advantageof this arrange is that the transmitted radar signal is turned off whenthe mobile is listening for reflections and hence is immune to Tx/Rxleakage. However, half duplex can complicate Rx/Tx scheduling whenmultiple reflectors with multiple distinct delays are present.

FIG. 14 is a functional block diagram of further transmit and receiveradar chains with another alternative antenna arrangement according to apresently disclosed embodiment. The embodiment of FIG. 14 is similar tothat of FIG. 13 but use an alternative antenna connection. In FIG. 14,both the transmit chain and the receive chain may be operatingsimultaneously in full duplex operation. The signals are routed to theantenna 1410 through a circulator 1408. The circulator 1408 can provide,for example, about 20 dB Rx-Tx isolation. Additional isolation may bedone by measuring an attenuated copy of the Tx signal and canceling itfrom the Rx signal using echo cancellation digital signal processing.Finally, additional isolation may be provided by a more than one chipdelay offset between the Tx and Rx DSSS signals due to the a reflector'slonger than one chip period built-in delay.

In a mobile using the arrangement of FIG. 14, even though both Tx and Rxcan operate at the same time, the Tx signal may be periodicallysilenced, for example, by turning off Tx gain stages and poweramplifiers. For example, if position reflectors have delays that clusteraround 3 microseconds, the Tx signal is transmitted for 3 microsecondsand turned off for 6 microseconds. While transmitting, the receiver inturned off. When the Tx is turned off, the Rx section receives andprocesses data. The 6 microseconds are used to receive the primary andsecondary reflections of DDR. Using DSSS signals, this can result in a−4.8 dB loss but can greatly simplify the design, for example, becauseit eliminates self-jamming.

FIG. 15 is a functional block diagram of location signal processingaccording to a presently disclosed embodiment. The location signalprocessing may be performed by mobile for position location. A transmitdigital signal processor 1507 supplies in-phase (TX_I) andquadrature-phase (TX_Q) values. The values may be supplied, for example,to the transmit chain 1205 of FIG. 12. A receive digital signalprocessor 1557 receives in-phase (RX_I) and quadrature-phase (RX_Q)values. The values may be received, for example, from the receive chain1255 of FIG. 12. The transmit digital signal processor 1507 and thereceive digital signal processor 1557 may be physically disjoint or maybe the same processor running both transmit and receive processingalgorithms concurrently. The transmit digital signal processor 1507 andthe receive digital signal processor 1557 are administered by anapplication processor 1559. In addition to processing the transmit andreceive data streams, in various combinations, the transmit digitalsignal processor 1507, the receive digital signal processor 1557, andthe application processor 1559 control other aspects of the transmit andreceive chains, such as LO frequency selection, Tx output power control,Rx input Low Noise Amplifier (LNA) gain and IP3 control, T/R switchcontrol, and transmit/receive scheduling among other RF chain bookkeeping operations. Using programmable block for processing gives themobile a great deal of flexibility in selecting the appropriatemodulation method, bandwidth, and center frequency for the transmittedradar signals along with the appropriate reciprocal processing in thereceive chain to detect range and AOA for a given the transmittedwaveform.

FIG. 16 is a functional block diagram of transmit location signalprocessing according to a presently disclosed embodiment. The blocks ofFIG. 16 may be used, for example, in the transmit digital signalprocessor 1507 of FIG. 15. The transmit location signal processing ofFIG. 16 includes a transmit pseudo-noise generator 1609 that generates apseudo-random sequence of {+1,−1} samples. The transmit pseudo-noisegenerator 1609 may generate the sequence by a maximal polynomial using alinear feedback shift register (LFSR). A transmit rotator 1604 canrotate transmit I and Q samples. An embodiment may use samples with BPSKmodulation on the I channel and nothing on the Q channel. These I and Qsamples are rotated in the transmit rotator 1604 before passing on tothe next stage of the Tx chain. The transmit rotator 1604 is used toshift the center frequency of the transmitted signal. The rotatorreceives angle increments of rotation per sample from a direct digitalsynthesizer (DDS) 1606 driven by a transmit ramp generator 1608. Thetransmit ramp generator 1608 may be programmable to start from zerofrequency offset to an offset equal to half the sampling frequency atbase band. A main processor of the system can program the transmitpseudo-noise generator 1609 with the appropriate polynomial to use andalso load the transmit ramp generator 1608 with the end frequency shiftand the speed of the frequency shift (Hz/second). Then the mainprocessor can issue a start command where the transmit pseudo-noisegenerator 1609 and the transmit ramp generator 1608 commence operationssynchronously. Thus, each instantaneous shift frequency is tightlyaligned to a PN phase of the PN code.

The transmit location signal processing also includes a receivepseudo-noise generator 1659, a receive ramp generator 1558, and areceive DSS 1656 to be used on the Rx signal chain side. The receivepseudo-noise generator 1659 is loaded with the same polynomial as thetransmit pseudo-noise generator 1609, and the receive pseudo-noisegenerator 1659 is started at the same instant as the transmitpseudo-noise generator 1609. However, the receive pseudo-noise generator1659 is loaded with a different initial condition that results in thegenerated Rx PN sequence lagging behind the Tx PN sequence by adetermined number of samples. Similarly, the receive ramp generator 1558is programmed with the same target frequency shift and the same rate ofshift as the transmit ramp generator 1608 and is also startedsynchronously with the transmit ramp generator 1608. However, thereceive ramp generator 1558 is programmed with different initialcondition that causes the ramp on the Rx side to lag behind the Tx ramp.The receive ramp generator 1558 drives the receive DSS 1656 to generatereceive rotation coefficients. Both the Rx PN sequence and the output ofthe receive digital signal processor 1557 are used in processing the Rxsamples. The reason for the delay between the Rx PN and ramp samplesrelative to the Tx samples is to account for the round trip delay of thereflected radar signal. In one implementation, the generator is run at arate of 10 million chips per second (10 MCPS). It is important to notethat the mobile can change this rate at will, since the mobile bothgenerates and detects the signal without external signal alterations andhence can use whatever is needed based on present condition as long asit is self-consistent from Tx to Rx.

FIG. 17 is a functional block diagram of receive location signalprocessing according to a presently disclosed embodiment. The blocks ofFIG. 17 may be used, for example, in the receive digital signalprocessor 1557 of FIG. 15. The receive location signal processingincludes a sample rotator 1754, a bank of I and Q correlators 1765, anda FIFO 1767. Rx RF samples are first rotated by the sample rotator 1754according values used to generate a corresponding transmitted signal.For example, when used with the transmit location signal processing ofFIG. 16, the sample rotator 1754 may receive rotation values from thereceive DSS 1656.

The rotated samples are then fed to the correlators 1765 to correlatewith the Rx PN sequence. The correlators and their correspondingrotators can use the information generated in the transmit digitalsignal processor to look for multiple reflector echoes. When used withthe transmit location signal processing of FIG. 16, the Rx PN sequencemay be supplied by the receive pseudo-noise generator 1659. In anexample implementation, there are 32 I and 32 Q correlators. Each of thecorrelator pairs correlates the incoming Rx samples with shifted copiesof the Rx PN sequence. Each PN sequence cycle, the integration resultsof the correlators are dumped to the main processor through the FIFO1767, for example, using a direct memory access (DMA) mechanism. A largevalue in a correlator sum indicates a reflector signal at the PN offsetused by that correlator and corresponds to a range measurement. Also,the relative magnitude between I and Q path correlators using the samePN and rotator values yields the carrier phase of this echo by computingthe arctangent of Q/I. To get finer time resolution, the sample rate ofthe Rx samples may be kept at a multiple of the PN chipping rate. In anembodiment, an Rx sample rate is used that is 8 times higher than the TxPN chipping rate. For an example implementation using a 10 MCPS PN rate,the Rx RF signal is sampled and processed at 80 MSPS.

Periodically, the main processor gets the results of the correlators1765. The processor searches for peaks (maxima) among these correlatorsums. Pronounced values of I²+Q² indicates the presence of an echo atthe corresponding round trip time slot. The processor then computes thecarrier phase of this pronounced echo and notes the frequency shift atwhich this carrier phase occurred. After processing a number ofcorrelator dumps, the processor can build a table of carrier phaseversus shift frequency at which the phases where observed for eachidentified echo.

For a given identified echo, the following processing can be performed.Assuming that the transmitted radar signal starts at center frequency f₀and sweeps to f₁, from the carrier phase versus shift frequency tablefor an identified echo, the processing is able to measure the totalphase shift the echo experienced while sweeping from f₀ to f₁. The tableentries are used instead of just the first and last values in order tokeep track of the total phase shift including counting full cycleshifts. Because each identified primary echo will have another secondaryecho due to DDR, there is enough information to calculate the free airrange between the mobile and the reflector to a fraction of a carriercycle. Assuming that while sweeping from f₀ to f₁, the primary echoexperiences a cyc_slip_(primary) carrier cycle slip, wherecyc_slip_(Primary) may not be an integer. Also, assuming that thesecondary echo experiences a cyc_slip_(secondary) carrier cycle slip.From this, the total round trip carrier cycles at frequency f_(n) fromTx to Rx is equal to number of cycles in the delay element,cyc_(delay,n), plus number of cycles in free air round trip,cyc_(air,n). For the secondary reflection, the round trip cycles valueis equal to number of cycles in free air, cyc_(air,n), plus two timesthe delay element cycles, 2×cyc_(delay,n). If L is the free air roundtrip to the reflector, cyc_(air,n)=L/λ_(n)=L F_(n)/c, where λ_(n) is thecarrier wavelength at center frequency f_(n), and c is the speed oflight in air. Also, cyc_(delay,n)=element_delay×f_(n). Practically we donot know cyc_(delay,n) or cyc_(air,n) but we can measurecyc_slip_(Primary) and cyc_slip_(secondary) accurately in the mobileduring a given f₀ to f₁ sweep. Note that,cyc_slip_(Primary)=(cyc_(air,1)−cyc_(air,0))+(cyc_(delay,1)−cyc_(delay,0)),cyc_slip_(secondary)=(cyc_(air,1)−cyc_(air,0))+2×(cyc_(delay,1)−cyc_(delay,0)),cyc_(air,1) =L(f ₁ −f ₀)/c+cyc_(air,0), andcyc_(delay,1)−cyc_(delay,0)=element_delay×(f ₁ −f ₀)

From the above equations, a positioning process can calculate L and thedelay of the delay element. The range to the reflector R=L/2 and thereflector element delay is used to identify the reflector from which theecho was produced.

The above method is useful since it eliminates any dependence of rangemeasurements on delay element manufacturing and environmental variationsand eliminates the need for constant reflector calibration. Alsodetermining delay element delays of an echo to the accuracy at carrierphase level aids in identifying the corresponding reflector from which areflected signal has been detected. For a system operating at 2.4 GHz,each carrier cycle spans 12 cm. Depending on available SNR, it becomespossible to obtain centimeter level range accuracy as well as identifydistinct reflectors through their differing delay element delays at thenanosecond level. This allow for the distinction among hundreds ofdistinct reflectors within the same vicinity.

The above outlines how narrow frequency shifting can measure the rangesand delays of reflectors. Prior to performing this narrow frequencysweep, a broad frequency sweep may be done. This broad frequency sweepis designed to detect the optimal frequency for a frequency scanned typereflector. Once the maximum reflection frequency is detected, a narrowfrequency sweep around that point is performed to measure the range anddelay. This may only done in locations with sparse number of reflectorsand hence the use of frequency swept reflectors yielding AOA becomesvaluable. In environments where more than three reflectors aredetectable, AOA can be ignored requiring no broad frequency sweep.

To support a multiuser environment and reduce confusing reflections,active power control can be implemented. Active power controlcontinuously uses the received reflections measured power to adjust theoutput Tx power. This results in the minimum RF power being transmittedby one mobile to determine its position. In addition, using different ortime shifted LFSR polynomials for each mobile insures that each mobileonly detects reflections caused by its own generated radar signal.

Most description herein have assumed that the mobile is stationary (ornearly so) and hence the effects of Tx to Rx frequency shift due toDoppler can be ignored. This is true for majority of indoor situationssince a position fix can take less than one millisecond. Even at anunrealistic 30 km/hr indoor speed, Doppler at 2.4 GHz is less than 25degrees every millisecond. However, a positioning system can correct forthe effect of Doppler by measuring it from cycle slipping rate whilefixing the carrier frequency (not sweeping) for some period.

An overview of an installation process will now be provided. Duringinstallation, the reflector data (e.g., in the server 194 of FIG. 1) isuploaded. The reflector data may be uploaded, for example, through theinternet by the installer, who has been given privileges to add to oralter the server data base. The reflector data may be uploaded using anyof a number of Internet connected computing devices, for example, usinga browser. After affixing position reflectors in a given area, thereflector data to be uploaded to the server can be obtained in variousways. The information may be obtained, for example, in a commercialsetting, from accurate building blue prints and surveying equipment todetermine the location and orientation of every reflector. Also,information specific to a given reflector could be downloaded from amanufacturer data sheet. All this information can be collected anduploaded to the server by the installer.

Alternately or additional, a mobile with positioning ability can be usedto make position determinations while located at a number of previouslysurveyed points relative to a local coordinate system. If sufficientdata points are taken, the server can determine the location of eachreflector it receives an echo from along with their measuredcorresponding element delays. Only four non-planar points isgeometrically sufficient for all detectable reflectors.

To relate the geometry of the mobile location and reflectors to thephysical walls and objects in the room the following methodology can beused. A software package can create a 3D model of a room includingobjects within the room from number of photographs of the room. If themeasuring points for locating the reflectors are identified within someof the photographs, then the 3D map of the room has a fixed relationshipto the coordinate system of the reflectors and hence the movement of themobile can be plotted in 3D within the 3D model of the room. This canprovide a great aid for application developers to use the data generatedaccording to the position systems and method disclosed herein.

A combination installation process operates as follows. After installingthe reflectors, a mobile with positioning and equipped with a digitalcamera is made to take a number of photographs of the internals of theroom. Each time a photo is snapped, the mobile measures ranges andreflector delays for all the reflectors it detects. After a few points,the mobile can locate the reflectors locations relative to each otherand the mobile. This information is used while the 3D model of the roomis determined by the taken photographs from which a 3D model of the roomis built while at the same time overlaying on top of the model thecoordinate system of the installed reflectors.

Alternatively or additionally, the reflector data may be obtained, forexample, in residential homes, by imaging. Mobile application softwaremay be used to produce a 3D room map by holding the mobile upright andscanning with the camera in the mobile all the walls of the room. Inorder to locate the reflectors, each of the reflectors may have an areathat has been covered with retro-reflective optical tape. This tapeshows up very brightly in the camera if illuminated by the camera flash.So during the scanning of the room with the camera, the flash of themobile is kept on. Whenever a reflector comes into frame, the opticalretro-reflective tape shows up very brightly in the camera and isidentified as a reflector and its location within the room noted. Thistape can be made with various colors or color stripes so that the mobilecould identify which of surrounding reflector it is. A bar code on thereflector would be scanned a priori so that the mobile is able to getall of the reflector RF characteristics and optical tape color from asimple bar code scan. After collecting this information about the roomgeometry, the location and orientation of each reflector and thereflector characteristics, the mobile application uploads thisinformation to the server to be available to other mobiles that need todetermine their location within that room.

Upload privileges may be given based on location of origination. Forexample, if a mobile is attempting to upload reflector information tothe server about a given room, the server would only accept thisinformation if it came from a router that is physically present near theroom. A password may also provisioned for added security. As mentionedabove, any mobile that succeeds in determining its location, cananonymously upload to the server the location it has determined, fromwhat reflector assumptions, and with what degree of confidence. Theserver uses this information that is constantly being uploaded bylocation determining mobiles to continually improve and refine theaccuracy of its resident data base. The mobile proceeds to determine itslocation by detecting and processing the radar signal reflections fromclose by reflectors. It uses the information available from the serverthat was uploaded to the server as mentioned previously in thecalculation. It then compares the determined position to its actualsurveyed position and uploads corrections to the server information inorder to improve accuracy of subsequent position determinations by othermobiles.

Indoor positioning described herein relies on the detection of radarreflected signals to determine physical location. Detecting radarreflections can be done by using a matched filter in the case of a CW orLFM type radar signal. In case that the transmitted radar signal is DSSSencoded, a RAKE receiver can be used to detect the weak reflections. Inboth cases, the phase of the reflection is determined through varioustechniques that track the received signal carrier frequency phase aftereither pulse compression, in the case of CW or LFM, or PRN de-spreadingis performed. The integer carrier ambiguity is resolved according to thetechniques described above, such as short duration pulsing, orincreasing the bandwidth through LFM or DSSS methods with coherentdemodulation. The coherent processing is made easy due to the commontransmit and receive frequency reference used.

Although this disclosure focuses on a mobile determining its locationwithin an indoor environment, the same or similar techniques can be usedto determine a mobile's location within an outdoor environment. Thistype of outdoor positioning may be used, for example, in “urban canyons”where narrow streets and tall building reduce the availability andaccuracy of GPS based location determination due to GPS signal blockingand multipath. Additionally, the disclosed positioning systems andmethod may be adapted to locate drones including determine relativepositions of multiple drones. Various drones may include positionreflectors, positioning as described for a mobile device, or both. Sucha system may be used for collision avoidance.

The disclosed positioning can be performed by a mobile device usingcircuitry already found in mobile devices. The circuitry can be sharedbetween performing their original function and the new positionlocations functions. A number of radar signal modulations that can beused with the reflectors and how to resolve the carrier integerambiguity to yield very accurate range measurement have been disclosed.The mobile architecture in an embodiment can generate and process all ofthe waveforms described herein and is flexible enough to alter the radarwave modulation and characteristics on the fly as needed. Finally, how asystem can be installed and how the server information can be refined inboth commercial as well as residential settings is disclosed.

FIG. 18 is a flowchart a process for determining position locationaccording to a presently disclosed embodiment. To provide a specificexample, the process will be described as executed by the mobile device185 in the system of FIG. 1. However, the process may be performed usingany suitable apparatus. Additionally, further details of many steps aredescribed above.

When the mobile wishes to determine its position, in step 1802, itretrieves its last location and collects environmental data about itssurroundings. In step 1803, the last location and environmental data aresent to the server 194. In step 1804, the mobile receives, from theserver, reflector data for position reflectors in its vicinity. Themobile may also receive information on how it should be perform thepositioning, for example, the allowed bandwidth, RF power, and timeslots for the transmitted radar signal.

In step 1805, the mobile determines the radar signal characteristics itwill use to begin its position determination. For example, the mobilecan determine the radar signal modulation scheme, bandwidth, outputpower, chipping rate, and timings. These characteristics may bedetermined based on a desired position accuracy.

In step 1806, the mobile can load and initialize transmit and receiveLFSR and ramp generators, for example, as described with reference toFIG. 16. In step 1807, the mobile start the transmit and receive LFSRand ramp generators and radiates the generated radar signal. In step1808, the mobile processes signals received in response to thetransmission of step 1807. For example, the process may look forcorrelation peaks to detect echoes and also measure cycle slips whileshifting center frequency. The mobile then determines, in step 1810, ifsufficient peaks are found in the correlation data. This determinationmay be based on a desired position accuracy and on what previousprocessing has been performed. If sufficient peaks are found, theprocess continues to step 1817. If sufficient peaks are not found, theprocess continues to step 1811 where it changes the center frequency ofthe radar signal and then returns to step 1806. If sufficient peaks arenot found and process has iterated steps 1806-1810 over a range ofcenter frequencies, the process continues to step 1812. In step 1812,the process increases the transmit power and then returns to step 1806.

In step 1817, the process computes ranges to reflectors using thecorrelation peaks. The process can then compute its location. In step1818, the process determines if it is detecting a frequency sweptreflector. In this case, the process continues to step 1819 where it candetermine AOA and further determine its location. Otherwise, the processcontinues to step 1827.

In step 1827, the mobile has determined its location. The mobile cansend the determined location and also detected reflector delays to theserver, which can then update its database of reflector data. Theprocess then continues to step 1829 where it remains until a positionupdate is desired. In this event, the process returns to step 1805.

The process of FIG. 18 may be modified by adding, omitting, reordering,or altering steps. Additionally, steps may be performed concurrently anda step that occurs after another step need not be immediately after.

The foregoing systems and methods and associated devices and modules aresusceptible to many variations. Additionally, for clarity and concision,many descriptions of the systems and methods have been simplified. Forexample, the figures generally illustrate one or few of each type ofdevice, but a communication system may have many of each type of device.Additionally, features of the various embodiments may be combined incombinations that differ from those described above.

As described in this specification, various systems and methods aredescribed as working to optimize particular parameters, functions, oroperations. This use of the term optimize does not necessarily meanoptimize in an abstract theoretical or global sense. Rather, the systemsand methods may work to improve performance using algorithms that areexpected to improve performance in at least many common cases. Forexample, the systems and methods may work to optimize performance judgedby particular functions or criteria. Similar terms like minimize ormaximize are used in a like manner.

Those of skill will appreciate that the various illustrative logicalblocks, modules, units, and algorithm steps described in connection withthe embodiments disclosed herein can often be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, and steps have been describedabove generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular constraints imposed on the overall system. Skilled personscan implement the described functionality in varying ways for eachparticular system, but such implementation decisions should not beinterpreted as causing a departure from the scope of the invention. Inaddition, the grouping of functions within a unit, module, block, orstep is for ease of description. Specific functions or steps can bemoved from one unit, module, or block without departing from theinvention.

The various illustrative logical blocks, units, steps and modulesdescribed in connection with the embodiments disclosed herein can beimplemented or performed with a processor, such as a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor canbe a microprocessor, but in the alternative, the processor can be anyprocessor, controller, microcontroller, or state machine. A processorcan also be implemented as a combination of computing devices, forexample, a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Furthermore, in addition toexecuting instructions, a processor may include specific purposehardware to accomplish some functions.

The steps of any method or algorithm and the processes of any block ormodule described in connection with the embodiments disclosed herein canbe embodied directly in hardware, in a software module executed by aprocessor, or in a combination of the two. A software module can residein RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory,registers, hard disk, a removable disk, a CD-ROM, or any other form ofstorage medium. An exemplary storage medium can be coupled to theprocessor such that the processor can read information from, and writeinformation to, the storage medium. In the alternative, the storagemedium can be integral to the processor. The processor and the storagemedium can reside in an ASIC. Additionally, device, blocks, or modulesthat are described as coupled may be coupled via intermediary device,blocks, or modules. Similarly, a first device may be described astransmitting data to (or receiving from) a second device when there areintermediary devices that couple the first and second device and alsowhen the first device is unaware of the ultimate destination of thedata.

The above description of the disclosed embodiments is provided to enableany person skilled in the art to make or use the invention. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles described herein can beapplied to other embodiments without departing from the spirit or scopeof the invention. Thus, it is to be understood that the description anddrawings presented herein represent particular aspects and embodimentsof the invention and are therefore representative examples of thesubject matter that is broadly contemplated by the present invention. Itis further understood that the scope of the present invention fullyencompasses other embodiments that are, or may become, obvious to thoseskilled in the art and that the scope of the present invention isaccordingly not limited by the descriptions presented herein.

What is claimed is:
 1. A positioning system, comprising: one or moreposition reflectors, each of the one or more position reflectorsconfigured to reflect radar signals, each reflected radar signalincluding a primary reflection delayed by an internal delay of theposition reflector and a secondary reflection delayed from the primaryreflection; a server storing reflector data associated with the one ormore position reflectors; and a mobile device configured to receive thereflector data from the server and to transmit a radar signal andprocess reflections of the transmitted radar signal to determine alocation of the mobile device.
 2. The positioning system of claim 1,wherein the secondary reflection is delayed from the primary reflectionby an integer multiple of the internal delay of the position reflector.3. The positioning system of claim 1, wherein the reflector data storedby the server includes: locations of the one or more positionreflectors; orientations of the one or more position reflectors;internal delays of the one or more position reflectors; delays fromprimary to secondary reflections of the one or more position reflectors;power levels of the primary and secondary reflections of the one or moreposition reflectors; and frequency characteristics of the one or moreposition reflectors.
 4. The positioning system of claim 1, wherein theserver is configured to control radiation power, bandwidth, duration,and center frequency of the radar signal transmitted by the mobiledevice.
 5. The positioning system of claim 1, wherein at least one ofthe one or more position reflectors comprises: an antenna; a delayelement; and a matching network coupling the antenna to the delayelement, the matching network transforming an impedance of the antennato an impedance of the delay element.
 6. The positioning system of claim5, wherein the matching network mismatches the impedance of the antennato the impedance of the delay element thereby causing the secondaryreflection from the position reflector.
 7. The positioning system ofclaim 1, wherein at least one of the one or more position reflectorscomprises: an antenna; a delay element; a circulator having a firstterminal for coupling to the antenna, a second terminal coupled to afirst terminal of the delay element, and a third terminal coupled to asecond terminal of the delay element; and a matching network couplingthe antenna to the circulator, the matching network transforming animpedance of the antenna to an impedance of the delay element.
 8. Thepositioning system of claim 7, wherein the matching network mismatchesthe impedance of the antenna to the impedance of the delay elementthereby causing a secondary reflection from the position reflector. 9.The positioning system of claim 1, wherein at least one of the one ormore position reflectors comprises: an array of antennas; a plurality oftransmission paths sequentially coupling the array of the antennas; adelay element; and a matching network coupling a first one of theplurality of transmission paths to the delay element, the matchingnetwork transforming an impedance of the array of antennas to animpedance of the delay element.
 10. The positioning system of claim 9,wherein the matching network mismatches the impedance of the array ofantennas to the impedance of the delay element thereby causing thesecondary reflection from the position reflector.
 11. The positioningsystem of claim 9, further comprising a spreading lens disposed todiffract reflections from the position reflector.
 12. The positioningsystem of claim 1, wherein at least one of the one or more positionreflectors comprises: a local oscillator producing a radio-frequencysignal; and an array of reflector elements, each of the reflectorelements including an antenna, a delay element, a matching networkcoupling the antenna to a first port of the delay element, the matchingnetwork transforming an impedance of the antenna to an impedance of thedelay element, and a reflector mixer coupled to a second port of thedelay element and configured to mix a signal from the delay element withthe radio-frequency signal from the local oscillator.
 13. Thepositioning system of claim 12, wherein the reflector mixer comprises: amixer configured to produce a mixed signal from a first input and asecond input, the second input coupled to the radio-frequency signalfrom the local oscillator; and a circulator having a first terminalcoupled to the second port of the delay element, a second terminalcoupled to the first input of the mixer, and a third terminal coupled tothe mixed signal from the mixer.
 14. The positioning system of claim 12,further comprising a phase shifter configured to modulate theradio-frequency signal with a data sequence that identifies the positionreflector before the radio-frequency signal is mixed at the reflectormixers.
 15. The positioning system of claim 12, wherein the matchingnetworks mismatch the impedance of the antenna to the impedance of thedelay element thereby causing the secondary reflection from the positionreflector.
 16. The positioning system of claim 12, wherein the delayelements include digital delay lines.
 17. The positioning system ofclaim 1, wherein at least one of the one or more position reflectorscomprises: a first reflector arranged to reflect radar signal sensitiveto a first angle; and a second reflector arranged to reflect radarsignal sensitive to a second angle orthogonal to the first angle.
 18. Adevice for location positioning, the device comprising: aradio-frequency front end coupled one or more antennas; and a processorcoupled to the radio-frequency front end and configured to supply aradar signal to the radio-frequency front end for transmission, receive,from the radio-frequency front end, signals reflected from one or moreposition reflectors, and process the received signals to determine alocation of the device, the processing including determining a delayfrom a primary reflection to a secondary reflection.
 19. The device ofclaim 18, wherein the radio-frequency front end comprises: a transmitchain including digital-to-analog converters to convert in-phase andquadrature transmit signals from digital to analog form, up-conversionmixers to convert the analog in-phase and quadrature transmit signals toradio frequency, a summer to combine the radio-frequency in-phase andquadrature transmit signals to form a transmit signal, and an amplifierand a transmit band-pass filter to supply the transmit signal to the oneor more antennas; and a receive chain including an amplifier and areceive band-pass filter to process received signals from the one ormore antennas; down-conversion mixers to convert the processed receivedsignals to in-phase and quadrature receive signals, andanalog-to-digital converters to convert the in-phase and quadraturereceive signals from analog to digital form.
 20. The device of claim 18,wherein the radar signal supplied for transmission is modulated withdirect sequence spread spectrum (DSSS) modulation.
 21. The device ofclaim 20, wherein the processor is further configured to generate theradar signal using a pseudo-noise generator, a frequency shift rampgenerator, and a direct digital synthesizer, and a transmit samplerotator and to process the received signals using a receive samplerotator and a bank of correlators.
 22. The device of claim 18, wherein acenter frequency of the radar signal is shifted, and wherein theprocessor is further configured to determine angle-of-arrivalinformation based on processing of the received signals corresponding todifferent center frequencies.
 23. The device of claim 18, wherein acenter frequency of the radar signal is swept between two frequencies,and wherein the processor is further configured to count cycle slips forboth the primary reflection and the secondary reflection.
 24. The deviceof claim 23, wherein the processor is further configured to utilize thecounted cycle slips to determine an internal delay of the associatedposition reflector.
 25. The device of claim 23, wherein the processor isfurther configured to utilize the counted cycle slips to determine arange to the associated position reflector, wherein the range isdetermined to a fraction of a wavelength of the radar signal.
 26. Thedevice of claim 18, wherein the processor is further configured tosupply the radar signal having an on duration based on an expectedinternal delay of a position reflector and an off duration based on thedelay of the secondary reflection.
 27. The device of claim 18, whereinthe processor is further configured to utilize the delay from theprimary reflection to the secondary reflection to identify theassociated reflector.
 28. The device of claim 18, wherein the processoris further configured to decode a data sequence phase modulated on thereceived reflection and utilize the decode data sequence to identify theassociated reflector.
 29. A method for location positioning of anelectronic device, the method comprising: transmitting a direct sequencespread spectrum coded radar signal, the radar signal having a centerfrequency swept between two frequencies; receiving radar signals thatinclude reflections from a plurality of position reflectors, each of thereflections including a primary reflection delayed by an internal delayof the corresponding position reflector and a secondary reflectionfurther delayed by the internal delay of the corresponding positionreflector; correlating information detected in the received radarsignals with corresponding information used to form the transmittedradar signal to determine maxima in the reflections; detecting frequencyinformation and delay information about the reflection using thedetermined maxima in the reflections; determining information about theinternal delays of the position reflectors from the delay information;identifying the plurality of position reflectors using the determinedinternal delay information; determining a range to at least one of theplurality of position reflectors using the detected delay informationand the determined internal delay information; determining an angle ofarrival for the reflections from at least one of the plurality ofposition reflectors using the frequency information; and determining theposition of the electronic device using the range and the angle ofarrival.
 30. The method of claim 29, further comprising: affixing theplurality of position reflectors in an interior of a room; taking aplurality of digital images of the interior of the room while receivingreflections from the plurality of position reflectors; processing thereceived reflections to produce reflector measurements; combining theplurality of digital images and the reflector measurements to build amodel of the interior of the room, the model including reflector data,the reflector data including locations of the plurality of positionreflectors; and storing the reflector data at a server for using insubsequent location positioning.